Effects of testosterone on hippocampal CA1 spine synaptic density in the male rat are inhibited by fimbria/fornix transection

Impacts of sex differences on optogenetic, chemogenetic, and calcium-imaging tools

Technical innovation in neuroscience introduced powerful tools for measuring and manipulating neuronal activity via optical, chemogenetic, and calcium-imaging tools. These tools were initially tested primarily in male animals but are now increasingly being used in females as well. In this review, we consider how these tools may work differently in males and females. For example, we review sex differences in the metabolism of chemogenetic ligands and their downstream signaling effects. Optical tools more directly alter depolarization or hyperpolarization of neurons, but biological sex and gonadal hormones modulate synaptic inputs and intrinsic excitability. We review studies demonstrating that optogenetic manipulations are sometimes consistent across the rodent estrous cycle but within certain circuits; manipulations can vary across the ovarian cycle. Finally, calcium-imaging methods utilize genetically encoded calcium indicators to measure neuronal activity. Testosterone and estradiol can directly modulate calcium influx, and we consider these implications for interpreting the results of calcium-imaging studies. Together, our findings suggest that these neuroscientific tools may sometimes work differently in males and females and that users should be aware of these differences when applying these methods.

Effects of membrane androgen receptor binding on synaptic plasticity in primary hippocampal neurons

Androgens play an important role in the regulation of hippocampal synaptic plasticity. While the classical molecular mechanism of androgen's genomic activity is their binding to intracellular androgen receptors (iARs), they can also induce rapid non-genomic effects through specific membrane androgen receptors (mARs). In this study, we aimed to localize and characterize these mARs in primary rat hippocampal neurons. Specific punctate fluorescent signals on the cell surface, observed by testosterone-fetal bovine serum albumin conjugated fluorescein isothiocyanate (T-BSA-FITC), indicated the presence of mARs in hippocampal neurons. T-BSA-FITC binding to the cell membrane was incompletely blocked by the iAR-antagonist flutamide, and mAR binding site was competitively bound by free testosterone (T). Most neurons expressing androgen membrane binding sites are glutamatergic (excitatory), although several are γ-aminobutyric acid (GABA)ergic (inhibitory). Confocal microscopy and live-cell imaging techniques were used to observe the real-time rapid effects of androgens on hippocampal dendritic spine morphology. Immunofluorescence cell staining was used to observe their effects on the postsynaptic density protein 95 (PSD95) and synapsin (SYN) synaptic markers. While androgens did not cause a short-term increase in dendritic spine density of rat primary hippocampal neurons, they promoted the transformation of dendritic spines from thin to mushroom, promoted dendritic spine maturation, increased dendritic spine surface area, and rapidly increased PSD95 and SYN expression in the primary hippocampal neurons. Hippocampal synaptosomes were prepared using the Optiprep and Percoll density gradient two-step centrifuge methods, and the gene expression profiles of the synaptosomes and hippocampus were compared using a gene chip; PSD95 mRNA expression was detected by reverse transcription-polymerase chain reaction. Several mRNAs were detected at the synaptic site, including PSD95. Finally, the Venus-PSD95 plasmid was constructed and transfected into HT22 cells, which is a mouse hippocampal neuronal cell line. The real-time effect of androgen on synaptic protein PSD95 was observed by fluorescence recovery after photobleaching experiments, which involved the translation process of PSD95 mRNA. In conclusion, our findings increased our understanding of how androgens exert the neuroprotective mechanisms on synaptic plasticity.

Comparison of neuroprotective effects of dihydrotestosterone, 17β-estradiol, and Pueraria mirifica herb extract on cognitive impairment in androgen deficient male rats

This study investigated the neuroprotective effects of dihydrotestosterone (DHT), 17β-estradiol (E₂), and Pueraria mirifica herb extract (PME; an alternative source of natural estrogens) on the (i) learning and memory in androgen-deficient male rats, and on the hippocampus expression levels of (ii) mRNA of genes associated with synaptic transmission and structure, neurofibrillary tangles, and amyloid plaques, and (iii) total and phosphorylated tau proteins. The four-month-old male rats were sham-operated or orchidectomized (ODX). The ODX rats were divided into four groups, and orally treated for 2 months with either 1 mL/d of distilled water or 100 mg/kg/d of PME; or subcutaneously injected with 1 mg/kg/d of DHT or 80 μg/kg/d of E₂. The impairment of spatial learning behavior and memory capacity in the ODX rats was prevented by DHT, E_2, and PME. Recovery of the orchidectomy-induced deterioration of the synaptic plasticity in the hippocampus of rats was ranked as E₂ ≥ PME > DHT. Both DHT and PME mitigated the increased Tau3 and Tau4 mRNA levels, and Tau-5 and P-Tau Ser³⁹⁶ protein levels more than E₂ (DHT ≥ PME > E₂). Only DHT tended to decrease App mRNA expression level. In conclusion, DHT showed a stronger efficacy for mitigation of the impaired spatial learning behavior and memory capacity in androgen-deficient male rats compared to E₂ and PME, and their mechanisms of action are slightly different.

The Sexual Dimorphic Synapse: From Spine Density to Molecular Composition

A synaptic sexual dimorphism is relevant in the context of multiple neurodevelopmental, neurodegenerative, and neuropsychiatric disorders. Many of these disorders show a different prevalence and progression in woman and man. A similar variance is also present in corresponding animal models. To understand and characterize this dimorphism in pathologies it is important to first understand sex differences in unaffected individuals. Therefore, sexual differences have been studied since 1788, first focusing on brain weight, size, and volume. But as these measures are not directly related to brain function, the investigation of sexual dimorphism also expanded to other organizational levels of the brain. This review is focused on sexual dimorphism at the synaptic level, as these specialized structures are the smallest functional units of the brain, determining cell communication, connectivity, and plasticity. Multiple differences between males and females can be found on the levels of spine density, synaptic morphology, and molecular synapse composition. These differences support the importance of sex-disaggregated data. The specificity of changes to a particular brain region or circuit might support the idea of a mosaic brain, in which each tile individually lies on a continuum from masculinization to feminization. Moreover, synapses can be seen as the smallest tiles of the mosaic determining the classification of larger areas.

Rodent models of stress and dendritic plasticity – Implications for psychopathology

Stress, as commonplace as it is, is a major environmental risk factor for psychopathology. While this association intuitively, anecdotally, and empirically makes sense, we are still very early in the process of understanding what the neurobiological manifestations of this risk truly are. Seminal work from the past few decades has established structural plasticity in the brain as a potential key mechanism. In this review we discuss evidence linking particularly chronic stress exposure in rodent models to plasticity at the dendrites, like remodeling of dendritic branches and spines, in a range of brain regions. A number of candidate mechanisms that seek to explain how stress influences neuroanatomy at this level have been proposed, utilizing in vivo, ex vivo and in vitro methods. However, a large gap still remains in our knowledge of how such dynamic structural changes ultimately relate to downstream effects such as altered affective and cognitive states relevant for psychopathology. We propose that future work expand our understanding of plasticity of specific stress-related brain circuits and cell-types. We also note that the vast majority of the work has been conducted solely on male rodents. The next big strides in our understanding of the neurobiology of psychopathology will require the inclusion of female subjects, as several studies have suggested both sex divergent and convergent features. By understanding plasticity, we can harness it. The growth of this body of knowledge will inform our efforts to improve the therapeutic options for stress-related psychopathology.

Sex-dependent responsiveness of hippocampal neurons to sex neurosteroids: A role of Arc/Arg3.1

Sex steroids, such as estradiol (E₂ ) and dihydrotestosterone (DHT), regulate hippocampal plasticity and memory in a sex-dependent manner. Because the activity-regulated cytoskeleton protein Arc/Arg3.1 is essential for long-term memory formation and synaptic plasticity, we investigated the expression of Arc/Arg3.1 with respect to its responsiveness to E₂ and DHT in male and female hippocampal neurons. For the first time, we show that, in hippocampal neurons, Arc/Arg3.1 expression is sex-dependently regulated by sex steroids. No difference in the expression between sexes was observed under control conditions. Using a quantitative real-time polymerase chain reaction, western blot analysis and quantitative immunoreactivity, upregulation of Arc/Arg3.1 protein expression was observed in specifically female hippocampal neurons after application of E₂ to the cultures. Conversely, upregulation of Arc/Arg3.1 was seen in specifically male neurons after application of DHT. A quantitative real-time PCR revealed that the sex-dependency was most pronounced on the mRNA level. Most importantly, the effects of E₂ in cultures of female animals were abolished when neuron-derived E₂ synthesis was inhibited. Our results point to a potentially important role of Arc/Arg3.1 regarding sex-dependency in sex steroid-induced synaptic plasticity in the hippocampus.

Androgen Effects on Neural Plasticity

Androgens are synthesized in the brain, gonads, and adrenal glands, in both sexes, exerting physiologically important effects on the structure and function of the central nervous system. These effects may contribute to the incidence and progression of neurological disorders such as autism spectrum disorder, schizophrenia, and Alzheimer's disease, which occur at different rates in males and females. This review briefly summarizes the current state of knowledge with respect to the neuroplastic effects of androgens, with particular emphasis on the hippocampus, which has been the focus of much of the research in this field.

On the role of sex steroids in biological functions by classical and non-classical pathways. An update

The sex steroid hormones (SSHs) play several roles in regulation of various processes in the cardiovascular, immune, muscular and neural systems. SSHs affect prenatal and postnatal development of various brain structures, including regions associated with important physiological, behavioral, cognitive, and emotional functions. This action can be mediated by either intracellular or transmembrane receptors. While the classical mechanisms of SSHs action are relatively well examined, the physiological importance of non-classical mechanism of SSHs action through membrane-associated and transmembrane receptors in the brain remains unclear. The most recent summary describing the role of SSHs in different body systems is lacking. Therefore, the aim of this review is to discuss classical and non-classical signaling pathways of testosterone and estradiol action via their receptors at functional, cellular, tissue level and to describe the effects on various body systems and behavior. Particular emphasis will be on brain regions including the hippocampus, hypothalamus, frontal cortex and cerebellum.

Behavioral sex differences in cocaine and opioid use disorders: The role of gonadal hormones

Females are more vulnerable than males to many aspects of cocaine use disorder. This vulnerability also translates to opioid use disorder, with females exhibiting stronger behavioral responses than males to drugs such as heroin and morphine. While there is evidence for many overlapping neural mechanisms underlying cocaine and opioid abuse, there is also a breadth of evidence indicating divergent effects of the drugs on synaptic plasticity. This makes it unclear whether the behavioral sex differences seen in substance use disorder across different drugs of abuse rely on the same mechanisms. Ovarian hormones have consistently been implicated as drivers of the behavioral sex differences in cocaine taking and seeking. While there are far fewer studies on the role of ovarian hormones in opioid use disorder, the existing data suggest that ovarian hormones may not drive these behavioral effects in the same manner as in cocaine use disorder. This review highlights evidence that behavioral sex differences in substance use disorder might be driven by different mechanisms depending on drug class.

Testosterone modulates structural synaptic plasticity of primary cultured hippocampal neurons through ERK - CREB signalling pathways

Although hippocampus-derived androgens play an important role in hippocampal synaptic plasticity, studies at the cellular level have received relatively less attention. Furthermore, the underlying signalling pathways associated with synaptic plasticity remain unclear. Results of the present study demonstrated that testosterone treatment of primary cultured rat hippocampal neurons resulted in a rapid increase in spine density, accompanied by the elevation of protein and messenger RNA levels of synaptophysin, developmentally regulated brain protein (Drebrin), and the N-methyl-D-aspartate receptor NR1 subunit. Testosterone treatment also increased the phosphorylation levels of extracellular-regulated protein kinase (ERK)1/2 and cAMP-responsive element binding protein (CREB), rather than p38 and Jun N-terminal kinase (JNK). U0126 significantly reversed the testosterone-mediated phosphorylation of CREB. Importantly, the increase in spine density was not induced by testosterone under U0126 treatment. These findings suggest that the ERK1/2-CREB signalling pathway plays an important role in testosterone-mediated rapid spinogenesis of cultured rat hippocampal neurons. Results of this study will be helpful in further clarifying the physiological function of testosterone and related signalling pathways in vitro.

Sex Differences in Synaptic Plasticity: Hormones and Beyond

Notable sex-differences exist between neural structures that regulate sexually dimorphic behaviors such as reproduction and parenting. While anatomical differences have been well-characterized, advancements in neuroimaging and pharmacology techniques have allowed researchers to identify differences between males and females down to the level of the synapse. Disparate mechanisms at the synaptic level contribute to sex-specific neuroplasticity that is reflected in sex-dependent behaviors. Many of these synaptic differences are driven by the endocrine system and its impact on molecular signaling and physiology. While sex-dependent modifications exist at baseline, further differences emerge in response to stimuli such as stressors. While some of these mechanisms are unifying between sexes, they often have directly opposing consequences in males and females. This variability is tied to gonadal steroids and their interactions with intra- and extra-cellular signaling mechanisms. This review article focuses on the various mechanisms by which sex can alter synaptic plasticity, both directly and indirectly, through steroid hormones such as estrogen and testosterone. That sex can drive neuroplasticity throughout the brain, highlights the importance of understanding sex-dependent neural mechanisms of the changing brain to enhance interpretation of results regarding males and females. As mood and stress responsivity are characterized by significant sex-differences, understanding the molecular mechanisms that may be altering structure and function can improve our understanding of these behavioral and mental characteristics.

Redefining neuroendocrinology: Epigenetics of brain-body communication over the life course

The brain is the central organ of stress and adaptation to stress that perceives and determines what is threatening, as well as the behavioral and physiological responses to the stressor, and it does so somewhat differently in males and females. The expression of steroid hormone receptors throughout the brain has broadened the definition of 'neuroendocrinology' to include the reciprocal communication between the entire brain and body via hormonal and neural pathways. Mediated in part via systemic hormonal influences, the adult and developing brain possess remarkable structural and functional plasticity in response to stress, including neuronal replacement, dendritic remodeling, and synapse turnover. This article is both an account of an emerging field elucidating brain-body interactions at multiple levels, from molecules to social organization, as well as a personal account of my laboratory's role and, most importantly, the roles of trainees and colleagues, along with my involvement in interdisciplinary groups working on this topic.

Sex differences in hippocampal area CA3 pyramidal cells

Numerous studies have demonstrated differences between males and females in hippocampal structure, function, and plasticity. There also are many studies about the different predisposition of a males and females for disorders where the hippocampus plays an important role. Many of these reports focus on area CA1, but other subfields are also very important, and unlikely to be the same as area CA1 based on what is known. Here we review basic studies of male and female structure, function, and plasticity of area CA3 pyramidal cells of adult rats. The data suggest that the CA3 pyramidal cells of males and females are distinct in structure, function, and plasticity. These sex differences cannot be simply explained by the effects of circulating gonadal hormones. This view agrees with previous studies showing that there are substantial sex differences in the brain that cannot be normalized by removing the gonads and depleting peripheral gonadal hormones. Implications of these comparisons for understanding sex differences in hippocampal function and dysfunction are discussed. © 2016 Wiley Periodicals, Inc.

Anti-androgenic effects of bisphenol-A on spatial memory and synaptic plasticity of the hippocampus in mice

Bisphenol-A (BPA) is a common environmental endocrine disruptor. Our recent studies found that exposure to BPA in both adolescent and adulthood sex-specifically impaired spatial memory in male mice. In this study, 11-week-old gonadectomied (GDX) male mice daily received subcutaneous injections of testosterone propionate (TP, 0.5mg/kg), TP and BPA (0.4 and 4mg/kg), or vehicle for 45days. The results of Morris water maze task showed that exposure to BPA did not affect the spatial memory of GDX mice but impaired that of sham (4mg/kg/day) and TP-treated GDX mice (0.4mg/kg/day). In addition, BPA reduced the level of testosterone (T) in the serum and brain of sham and TP-treated GDX mice. Exposure to BPA decreased the synaptic density and had an adverse effect on the synaptic interface of the hippocampus in sham and TP-treated GDX mice. The results of western blot analysis further showed that BPA (4mg/kg) reduced the levels of synaptic proteins (synapsin I and PSD-95) and NMDA receptor subunit NR2B in sham and TP-treated GDX mice. BPA decreased the phosphorylation of ERK1/2 but increased the phosphorylation of p38 in sham and TP-treated GDX mice. These results suggest that impairment of spatial memory and adverse effects on synaptic remodeling of hippocampal neurons in males after long-term BPA exposure is related to the anti-androgen effect of BPA. These effects of BPA may be associated with downregulated synaptic proteins and NMDA receptor through inhibiting ERKs and promoting the p38 pathways.

Expansion of mossy fibers and CA3 apical dendritic length accompanies the fall in dendritic spine density after gonadectomy in male, but not female, rats

Androgen loss is an important clinical concern because of its cognitive and behavioral effects. Changes in androgen levels are also suspected to contribute to neurological disease. However, the available data on the effects of androgen deprivation in areas of the brain that are central to cognition, like the hippocampus, are mixed. In this study, morphological analysis of pyramidal cells was used to investigate if structural changes could potentially contribute to the mixed cognitive effects that have been observed after androgen loss in males. Male Sprague-Dawley rats were orchidectomized or sham-operated. Two months later, their brains were Golgi-impregnated for morphological analysis. Morphological endpoints were studied in areas CA3 and CA1, with comparisons to females either intact or 2 months after ovariectomy. CA3 pyramidal neurons of orchidectomized rats exhibited marked increases in apical dendritic arborization. There were increases in mossy fiber afferent density in area CA3, as well as robust enhancements to dendritic structure in area CA3 of orchidectomized males, but not in CA1. Remarkably, apical dendritic length of CA3 pyramidal cells increased, while spine density declined. By contrast, in females overall dendritic structure was minimally affected by ovariectomy, while dendritic spine density was greatly reduced. Sex differences and subfield-specific effects of gonadal hormone deprivation on the hippocampal circuitry may help explain the different behavioral effects reported in males and females after gonadectomy, or other conditions associated with declining gonadal hormone secretion.

Gonadal Hormones Rapidly Enhance Spatial Memory and Increase Hippocampal Spine Density in Male Rats

17β-estradiol (E2) rapidly, within minutes, activates behaviors and cognition by binding to membrane estrogen receptors, activating cell signaling cascades and increasing dendritic spines. In female rodents, E2 enhances spatial memory within 2-4 hours, and spine density is increased in the CA1 area of the hippocampus within 30-60 minutes. Although chronic gonadal hormone treatments in male rats alter cognition and spines/spine synapses and acute hormone effects occur in hippocampal slices, effects of acute, in vivo hormone administration in males are unknown. Therefore, we assessed rapid effects of E2 (20 μg/kg) and testosterone (T) (750 μg/kg) on spatial memory using the object placement task and on hippocampal spine density using Golgi impregnation. Orchidectomized rats received hormones immediately after the training trial and were tested for retention 2 hours later. Vehicle-injected orchidectomized males spent equal time exploring objects in the old and new locations, but E2- or T-treated subjects spent more time exploring objects at the new location, suggesting enhanced memory. Both hormones also increased spine density in CA1, but not the dentate gyrus, by 20%-40% at 30 minutes and 2 hours after injections. This report is the first, to our knowledge, to show E2 and T enhancements of memory and spine density within such a short time frame in male rats.

Rapid increase of spines by dihydrotestosterone and testosterone in hippocampal neurons: Dependence on synaptic androgen receptor and kinase networks

Rapid modulation of hippocampal synaptic plasticity by locally synthesized androgen is important in addition to circulating androgen. Here, we investigated the rapid changes of dendritic spines in response to the elevation of dihydrotestosterone (DHT) and testosterone (T), by using hippocampal slices from adult male rats, in order to clarify whether these signaling processes include synaptic/extranuclear androgen receptor (AR) and activation of kinases. We found that the application of 10nM DHT and 10nM T increased the total density of spines by approximately 1.3-fold within 2h, by imaging Lucifer Yellow-injected CA1 pyramidal neurons. Interestingly, DHT and T increased different head-sized spines. While DHT increased middle- and large-head spines, T increased small-head spines. Androgen-induced spinogenesis was suppressed by individually blocking Erk MAPK, PKA, PKC, p38 MAPK, LIMK or calcineurin. On the other hand, blocking CaMKII did not inhibit spinogenesis. Blocking PI3K altered the spine head diameter distribution, but did not change the total spine density. Blocking mRNA and protein synthesis did not suppress the enhancing effects induced by DHT or T. The enhanced spinogenesis by androgens was blocked by AR antagonist, which AR was localized postsynaptically. Taken together, these results imply that enhanced spinogenesis by DHT and T is mediated by synaptic/extranuclear AR which rapidly drives the kinase networks. This article is part of a Special Issue entitled SI: Brain and Memory.

Androgen Modulation of Hippocampal Structure and Function

Androgens have profound effects on hippocampal structure and function, including induction of spines and spine synapses on the dendrites of CA1 pyramidal neurons, as well as alterations in long-term synaptic plasticity (LTP) and hippocampally dependent cognitive behaviors. How these effects occur remains largely unknown. Emerging evidence, however, suggests that one of the key elements in the response mechanism may be modulation of brain-derived neurotrophic factor (BDNF) in the mossy fiber (MF) system. In male rats, orchidectomy increases synaptic transmission and excitability in the MF pathway. Testosterone reverses these effects, suggesting that testosterone exerts tonic suppression on MF BDNF levels. These findings suggest that changes in hippocampal function resulting from declining androgen levels may reflect the outcome of responses mediated through normally balanced, but opposing, mechanisms: loss of androgen effects on the hippocampal circuitry may be compensated, at least in part, by an increase in BDNF-dependent MF plasticity.

Reduced hippocampal dendritic spine density and BDNF expression following acute postnatal exposure to di(2-ethylhexyl) phthalate in male Long Evans rats

Early developmental exposure to di(2-ethylhexyl) phthalate (DEHP) has been linked to a variety of neurodevelopmental changes, particularly in rodents. The primary goal of this work was to establish whether acute postnatal exposure to a low dose of DEHP would alter hippocampal dendritic morphology and BDNF and caspase-3 mRNA expression in male and female Long Evans rats. Treatment with DEHP in male rats led to a reduction in spine density on basal and apical dendrites of neurons in the CA3 dorsal hippocampal region compared to vehicle-treated male controls. Dorsal hippocampal BDNF mRNA expression was also down-regulated in male rats exposed to DEHP. No differences in hippocampal spine density or BDNF mRNA expression were observed in female rats treated with DEHP compared to controls. DEHP treatment did not affect hippocampal caspase-3 mRNA expression in male or female rats. These results suggest a gender-specific vulnerability to early developmental DEHP exposure in male rats whereby postnatal DEHP exposure may interfere with normal synaptogenesis and connectivity in the hippocampus. Decreased expression of BDNF mRNA may represent a molecular mechanism underlying the reduction in dendritic spine density observed in hippocampal CA3 neurons. These findings provide initial evidence for a link between developmental exposure to DEHP, reduced levels of BDNF and hippocampal atrophy in male rats.

Sex-specific effect of the anabolic steroid, 17α-methyltestosterone, on inhibitory avoidance learning in periadolescent rats

The illicit use of anabolic androgenic steroids (AAS) has gained popularity among adolescents in the last decade. However, although it is known that exposure to AAS impairs cognition in adult animal models, the cognitive effects during adolescence remain undetermined. An inhibitory avoidance task (IAT) was used to assess the effect of AAS (17α-methyltestosterone; 17α-meT--7.5 mg/kg) in male and female periadolescent rats. A single injection of 17α-meT immediately before the footshock produced significant impairment of inhibitory avoidance learning in males but not females. Generalized anxiety, locomotion, and risk assessment behaviors (RAB) were not affected. Our results show that exposure to a single pharmacological dose of 17α-meT during periadolescence exerts sex-specific cognitive effects without affecting anxiety. Thus, disruption of the hormonal milieu during this early developmental period might have negative impact on learning and memory.

Estrogen effects on the brain: actions beyond the hypothalamus via novel mechanisms

From its origins in how the brain controls the endocrine system via the hypothalamus and pituitary gland, neuroendocrinology has evolved into a science that now includes hormone action on many aspects of brain function. These actions involve the whole central nervous system and not just the hypothalamus. Advances in our understanding of cellular and molecular actions of steroid hormones have gone beyond the important cell nuclear actions of steroid hormone receptors to include signaling pathways that intersect with other mediators such as neurotransmitters and neuromodulators. This has, in turn, broadened the search for and identification of steroid receptors to include nonnuclear sites in synapses, dendrites, mitochondria, and glial cells, as well as cell nuclei. The study of estrogen receptors and estrogen actions on processes related to cognition, mood, autonomic regulation, pain, and neuroprotection, among other functions, has led the way in this new view of hormone actions on the brain. In this review, we summarize past and current work in our laboratory on this topic. This exciting and growing field involving many laboratories continues to reshape our ideas and approaches to neuroendocrinology both at the bench and the bedside.

Stress, sex, and neural adaptation to a changing environment: mechanisms of neuronal remodeling

The adult brain is much more resilient and adaptable than previously believed, and adaptive structural plasticity involves growth and shrinkage of dendritic trees, turnover of synapses, and limited amounts of neurogenesis in the forebrain, especially the dentate gyrus of the hippocampal formation. Stress and sex hormones help to mediate adaptive structural plasticity, which has been extensively investigated in the hippocampus and to a lesser extent in the prefrontal cortex and amygdala, all brain regions that are involved in cognitive and emotional functions. Stress and sex hormones exert their effects on brain structural remodeling through both classical genomic as well as non-genomic mechanisms, and they do so in collaboration with neurotransmitters and other intra- and extracellular mediators. This review will illustrate the actions of estrogen on synapse formation in the hippocampus and the process of stress-induced remodeling of dendrites and synapses in the hippocampus, amygdala, and prefrontal cortex. The influence of early developmental epigenetic events, such as early life stress and brain sexual differentiation, is noted along with the interactions between sex hormones and the effects of stress on the brain. Because hormones influence brain structure and function and because hormone secretion is governed by the brain, applied molecular neuroscience techniques can begin to reveal the role of hormones in brain-related disorders and the treatment of these diseases. A better understanding of hormone-brain interactions should promote more flexible approaches to the treatment of psychiatric disorders, as well as their prevention through both behavioral and pharmaceutical interventions.

3alpha-androstanediol, but not testosterone, attenuates age-related decrements in cognitive, anxiety, and depressive behavior of male rats

Some hippocampally-influenced affective and/or cognitive processes decline with aging. The role of androgens in this process is of interest. Testosterone (T) is aromatized to estrogen, and reduced to dihydrotestosterone (DHT), which is converted to 5α-androstane, 3α, 17α-diol (3α-diol). To determine the extent to which some age-related decline in hippocampally-influenced behaviors may be due to androgens, we examined the effects of variation in androgen levels due to age, gonadectomy, and androgen replacement on cognitive (inhibitory avoidance, Morris water maze) and affective (defensive freezing, forced swim) behavior among young (4 months), middle-aged (13 months), and aged (24 months) male rats. Plasma and hippocampal levels of androgens were determined. In experiment 1, comparisons were made between 4-, 13-, and 24-month-old rats that were intact or gonadectomized (GDX) and administered a T-filled or empty silastic capsule. There was age-related decline in performance of the inhibitory avoidance, water maze, defensive freezing, and forced swim tasks, and hippocampal 3α-diol levels. Chronic, long-term (1–4 weeks) T-replacement reversed the effects of GDX in 4- and 13-month-old, but not 24-month-old, rats in the inhibitory avoidance task. Experiments 2 and 3 assessed whether acute subcutaneous T or 3α-diol, respectively, could reverse age-associated decline in performance. 3α-diol, but not T, compared to vehicle, improved performance in the inhibitory avoidance, water maze, forced swim, and defensive freezing tasks, irrespective of age. Thus, age is associated with a decrease in 3α-diol production and 3α-diol administration reinstates cognitive and affective performance of aged male rats.

Activation of the androgen receptor alters the intracellular calcium response to glutamate in primary hippocampal neurons and modulates sarco/endoplasmic reticulum calcium ATPase 2 transcription

Androgens have been shown to have a number of effects on hippocampal function. Although androgen receptors (AR) are found at high levels in hippocampal neurons, the intracellular mechanisms responsible for androgen's actions are unknown. If androgens were capable of altering internal calcium concentration ([Ca(2+)](i)), they could influence a variety of intracellular signaling pathways, maintain neuronal homeostasis and Ca(2+) induced excitotoxicity. In the present study, calcium imaging was used to measure the [Ca(2+)](i) in rat primary hippocampal neurons treated with either the AR agonist dihydrotestosterone (DHT), DHT+flutamide (AR antagonist), flutamide alone, or vehicle for 24 h and subsequently presented with an excitatory glutamate stimulus. In the absence of glutamate stimulation, DHT treatment caused a significant upward shift in baseline [Ca(2+)](i) when compared with neurons from all other groups. Glutamate had a greater effect on [Ca(2+)](i) in DHT-treated neurons and DHT-treated neurons returned to baseline levels significantly faster than all other groups. Cyclopiazonic acid, an inhibitor of sarco/endoplasmic reticulum calcium ATPase (SERCA) had a larger response in DHT-treated neurons compared with controls, suggesting increased Ca(2+) stores in DHT-treated neurons. In all cases the effects of DHT were blocked by treatment with flutamide indicating an AR-mediated mechanism. To determine a possible mechanism by which AR activation could be influencing [Ca(2+)](i), SERCA2 mRNA levels were measured in primary hippocampal neurons. SERCA2 is inserted into the endoplasmic reticulum (ER) membrane and functions to rapidly pump [Ca(2+)](i) into the ER. Following treatment of primary hippocampal neurons with DHT, SERCA2 mRNA was increased, an effect that was blocked in the presence of flutamide. Taken together these results indicate that DHT, working through AR, causes an up-regulation of SERCA2, which increases the sequestering of [Ca(2+)](i) in the endoplasmic reticulum of hippocampal neurons. Such changes may allow the neurons to respond more robustly to a stimulus and recover more quickly following a highly stimulatory challenge.

Androgen regulation of beta-amyloid protein and the risk of Alzheimer's disease

Advancing age is the most significant risk factor for the development of Alzheimer's disease (AD), however the age-related changes that underlie this effect remain unclear. In men, one normal consequence of aging is a robust decline in circulating and brain levels of the sex steroid hormone testosterone. Testosterone depletion leads to functional impairments and increased risk of disease in androgen-responsive tissues throughout the body, including brain. In this review we discuss the relationship between age-related testosterone depletion and the development of AD. Specifically, we focus on androgen regulation of beta-amyloid protein (Abeta), the accumulation of which is a key initiating factor in AD pathogenesis. Emerging data suggest that the regulatory actions of androgens on both Abeta and the development of AD support consideration of androgen therapy for the prevention and treatment of AD.

Gonadal steroids and visuo-spatial abilities in adult males: implications for generalized age-related cognitive decline

The relationship between the gonadal steroids, testosterone and estrogen, and individual and group differences in performance on some cognitive tasks remains unclear but sex differences favoring males on some tests of visuo-spatial ability are large and robust. This aim of this review is to assess evidence for both organizational and activational effects of gonadal steroids as the principle cause of sex difference in visuo-spatial ability. Additionally, the implications of this relationship are discussed in the context of decreasing levels of gonadal steroids in aging males and psychological theories of generalized age-related cognitive decline. Based upon human and non-human research gonadal steroids have organizational effects on visuo-spatial ability in adulthood. Activational effects of gonadal steroids on visuo-spatial ability appear most dominant in older men and are necessary for maintaining optimal visuo-spatial ability; randomized clinical trials show that testosterone supplementation improves performance. Additionally, decreasing gonadal steroid levels in aging males may contribute to generalized age-related cognitive decline. Future supplementation studies in men should attempt to control for constituent abilities related to visuo-spatial task performance, and investigate interactions between dosage levels and baseline gonadal status. Further future animal research is required to investigate changes in gonadal steroid levels and their relationship to neurotransmitter systems, neural plasticity, and behavioral correlates.

Hippocampal formation: shedding light on the influence of sex and stress on the brain

The hippocampus is a malleable brain region that responds to external agents such as hormones and stressors. Investigations that began in our laboratories with the Golgi technique and an appreciation of hippocampal neuroanatomy at the light and electron microscopic levels have led us down a path that has uncovered unexpected structural plasticity in the adult brain along with unanticipated cellular and molecular mechanisms of this plasticity and of hormone mediation of these effects. This chapter reviews the history of discoveries in our two laboratories involving the actions of estradiol and stress hormones on neuronal structure and function and then discusses the insight to hormone-brain interactions that this has engendered. These discoveries have led us to a new view of brain structural plasticity and the role and mechanism of steroid hormone action involving both genomic and non-genomic pathways. This new view is consistent with the predictions of Cajal in his book "The Structure of Ammon's horn", 1892.

Testosterone modulates performance on a spatial working memory task in male rats

Gonadal hormones have been shown to modulate memory retention in female rats. The current experiments examine the role of testicular hormones in modulating the performance of male rats on two spatial water maze tasks. In the first study, castrated and intact rats were trained on the visible platform and hidden platform versions of the Morris water maze task. Castration did not affect performance on either version of this reference memory task with castrated and intact rats demonstrating similar performance both during acquisition and on post-training probe trials. In the second experiment, castrated and intact rats were tested on a delayed-matching-to-place version of the water maze. Rats received a series of trial pairs in the maze with a hidden platform located in the same pool location on the exposure and retention trials of each pair; between pairs of trials, however, the platform was repositioned to a novel pool location. The interval between trials was either 10- or 60-min and memory retention, taken as the difference between the pathlengths on the exposure and retention trials, declined as the interval increased. Relative to intact males, castrated males demonstrated impaired working memory retention at 60-min but not at 10-min retention intervals. This interval-dependent impairment in working memory retention was reversed by physiologic levels of testosterone replacement. These findings indicate that castration does not significantly affect acquisition or probe trial performance on a classic reference memory task but does impair spatial working memory retention, an effect that is reversed by exogenous testosterone.

Androgen effects on hippocampal CA1 spine synapse numbers are retained in Tfm male rats with defective androgen receptors

The effects of estradiol benzoate (EB), dihydrotestosterone (DHT), or the antiandrogen hydroxyflutamide on CA1 pyramidal cell dendritic spine synapses were investigated in adult male rats. To elucidate the contribution of the androgen receptor to the hormone-induced increase in hippocampal CA1 synapses, wild-type males were compared with males expressing the Tfm mutation, which results in synthesis of defective androgen receptors. Orchidectomized rats were treated with EB (10 microg/rat.d), DHT (500 mug/rat.d), hydroxyflutamide (5 mg/rat.d), or the sesame oil vehicle sc daily for 2 d and examined using quantitative electron microscopic stereological techniques, 48 h after the second injection. In wild-type males, DHT and hydroxyflutamide both induced increases in the number of spine synapses in the CA1 stratum radiatum, whereas EB had no effect. DHT almost doubled the number of synaptic contacts observed, whereas hydroxyflutamide increased synapse density by approximately 50%, compared with the vehicle-injected controls. Surprisingly, in Tfm males, the effects of EB, DHT, and hydroxyflutamide were all indistinguishable from those observed in wild-type animals. These observations demonstrate that Tfm male rats resemble normal males in having no detectable hippocampal synaptic response to a dose of EB that is highly effective in females. Despite the reduction in androgen sensitivity as a result of the Tfm mutation, hippocampal synaptic responses to both DHT and a mixed androgen agonist/antagonist (hydroxyflutamide) remain intact in Tfm males. These data are consistent with previous results suggesting that androgen effects on hippocampal spine synapses may involve novel androgen response mechanisms.

Androgen modulation of hippocampal synaptic plasticity

This review briefly summarizes recent developments in our understanding of the role of androgens in maintaining normal hippocampal structure. Studies in rats and vervet monkeys have demonstrated that removal of the testes reduces the density of synaptic contacts on dendritic spines of cornu ammonis 1 (CA1) pyramidal neurons. This effect is rapidly reversed by treatment with either testosterone or the non-aromatizable androgen dihydrotestosterone, suggesting that maintenance of normal synaptic density is androgen-dependent, via a mechanism that does not require intermediate estrogen biosynthesis. Similar effects of these androgens are observed in ovariectomized female rats, except that in the female the actions of testosterone include a substantial contribution from estrogen formation. The ability to stimulate hippocampal spine synapse density is not directly related to systemic androgenic potency: thus, weak androgens such as dehydroepiandrosterone exert effects that are comparable to those of dihydrotestosterone; while partial agonist responses are observed after injection of the synthetic antiandrogen, flutamide. These data provide a morphological counterpart to observations that androgens enhance cognitive function and mood state, suggesting that these effects may result at least in part from hippocampal neurotrophic responses. The unusual specificity of these responses raises the possibility that effects of androgens on the brain may be mediated via different mechanisms than the masculinizing actions of these steroids in non-neural androgen target organs.

Testosterone Reverses Ethanol-Induced Deficit in Spatial Reference Memory in Castrated Rats

The present study was designed to evaluate the effects of ethanol, testosterone and combination of ethanol and testosterone, on spatial reference memory and beta-endorphin (beta-EN) levels in castrated rats. Male Sprague-Dawley rats (120-150 g) were used in this study, Animals were castrated and ethanol, testosterone or combination of the drugs were administered to rats at 09:00 h. The drugs were administered after a training period of 5 days and spatial reference memory was evaluated for 7 days using the Morris water maze. One hour after the last injection, animals were sacrificed, their brains removed and dissected into cortex, hypothalamus, hippocampus and midbrain. The beta-EN levels in these brain regions were determined by radioimmunoassay. The time to find the platform (latency period) was significantly increased in ethanol-treated rats, indicating that ethanol induces deficit in spatial reference memory. On the other hand, testosterone administration improved spatial reference memory by significantly decreasing the latency period. In addition, there was a significant decrease in latency period in the animals treated with combination of ethanol and testosterone. Results also indicate that administration of ethanol resulted in a significant increase in beta-EN levels in the hippocampus and in the cortex while concurrent administration with testosterone abolished this increase. These findings clearly indicate that administration of testosterone did not only improve memory but also abolished the spatial memory deficit induced by ethanol in castrated rats.

Dihydrotestosterone increases hippocampal N-methyl-D-aspartate binding but does not affect choline acetyltransferase cell number in the forebrain or choline transporter levels in the CA1 region of adult male rats

Testosterone, acting through its androgenic metabolite 5alpha-dihydrotestosterone (DHT), can increase dendritic spine density in the CA1 region of the male rat hippocampus. The mechanisms mediating this increase in spines are presently unknown. In female rats, estrogen (E) has been shown to increase spine density, which is in part mediated by increases in N-methyl-d-aspartate (NMDA) receptors in the CA1 region and cholinergic forebrain inputs to the hippocampus. Whether similar mechanisms are responsible for the DHT-induced increase in spines in the male remains to be determined. In the first experiment, we used [(3)H]glutamate NMDA receptor binding autoradiography to assess whether DHT-treated males had higher NMDA receptor levels in the CA1 region of the hippocampus, compared with oil-treated males. In the second set of experiments, we used choline acetyltransferase (ChAT) in situ hybridization and immunohistochemistry to assess whether DHT could affect ChAT cell number in the forebrain. We also investigated the effect of DHT on hemicholinium-3-sensitive choline transporter levels in the CA1 region of the male hippocampus. We found that DHT significantly increased NMDA receptor binding in the CA1 region of males but had no effect on ChAT cell number in the forebrain or hemicholinium-3-sensitive choline transporter protein levels in the CA1 region. These data indicate that, similar to E-induced spinogenesis in females, DHT-induced increases in spine formation in males may require increases in NMDA receptors. However, unlike E-treated females, these data suggest that DHT does not influence cholinergic inputs to the hippocampus.

Ultrastructural evidence that androgen receptors are located at extranuclear sites in the rat hippocampal formation

Like estrogens in female rats, androgens can affect dendritic spine density in the CA1 subfield of the male rat hippocampus [J Neurosci 23:1588 (2003)]. Previous light microscopic studies have shown that androgen receptors (ARs) are present in the nuclei of CA1 pyramidal cells. However, androgens may also exert their effects through rapid non-genomic mechanisms, possibly by binding to membranes. Thus, to investigate whether ARs are at potential extranuclear sites of ARs, antibodies to ARs were localized by light and electron microscopy in the male rat hippocampal formation. By light microscopy, AR immunoreactivity (-ir) was found in CA1 pyramidal cell nuclei and in disperse, punctate processes that were most dense in the pyramidal cell layer. Additionally, diffuse AR-ir was found in the mossy fiber pathway. Ultrastructural analysis revealed AR-ir at several extranuclear sites in all hippocampal subregions. AR-ir was found in dendritic spines, many arising from pyramidal and granule cell dendrites. AR-ir was associated with clusters of small, synaptic vesicles within preterminal axons and axon terminals. Labeled preterminal axons were most prominent in stratum lucidum of the CA3 region. AR-containing terminals formed asymmetric synapses or did not form synaptic junctions in the plane of section analyzed. AR-ir also was detected in astrocytic profiles, many of which apposed terminals synapsing on unlabeled dendritic spines or formed gap junctions with other AR-labeled or unlabeled astrocytes. Collectively, these results suggest that ARs may serve as both a genomic and non-genomic transducer of androgen action in the hippocampal formation.

Sex differences in hippocampal estradiol-induced N-methyl-D-aspartic acid binding and ultrastructural localization of estrogen receptor-alpha

Estradiol increases dendritic spine density and synaptogenesis in the CA1 region of the female hippocampus. This effect is specific to females, as estradiol-treated males fail to show increases in hippocampal spine density. Estradiol-induced spinogenesis in the female is dependent upon upregulation of the N-methyl-D-aspartic acid (NMDA) receptor as well as on non-nuclear estrogen receptors (ER), including those found in dendrites. Thus, in the male, the inability of estradiol to induce spinogenesis may be related to a failure of estradiol to increase hippocampal NMDA receptors as well as a paucity of dendritic ER. In the first experiment, we sought to investigate this possibility by assessing NMDA receptor binding, using [(3)H]-glutamate autoradiography, in estradiol-treated males and females. We found that while estradiol increases NMDA binding in gonadectomized females, estradiol fails to modulate NMDA binding in gonadectomized males. To further investigate sex differences in the hippocampus, we conducted a second separate, but related, ultrastructural study in which we quantified ERalpha-immunoreactivity (ERalpha-ir) in neuronal profiles in the CA1 region of the hippocampus in intact males and females in diestrus and proestrus. Consistent with previous reports in the female, we found ERalpha-ir in several extranuclear sites including dendrites, spines, terminals and axons. Statistical analyses revealed that females in proestrus had a 114.3% increase in ERalpha-labeled dendritic spines compared to females in diestrus and intact males. Taken together, these studies suggest that both the ability of estrogen to increase NMDA binding in the hippocampus and the presence of ERalpha in dendritic spines may contribute to the observed sex difference in estradiol-induced hippocampal spinogenesis.

Androgens increase spine synapse density in the CA1 hippocampal subfield of ovariectomized female rats

The effects of androgen on the density of spine synapses on pyramidal neurons in the CA1 area of the hippocampus were studied in ovariectomized (OVX) adult female rats. Treatment of OVX rats with testosterone propionate (TP; 500 microg/d, s.c., 2 d) significantly increased spine synapse density (from 0.661 +/- 0.016 spine synapse/microm3 in OVX rats to 1.081 +/- 0.018 spine synapse/microm3 after TP treatment). A smaller, but still statistically significant, increase in synapse density (0.955 +/- 0.029 spine synapse/microm3) was observed in OVX animals after treatment with the nonaromatizable androgen dihydrotestosterone (DHT; 500 microg/d, s.c., 2 d). Administration of 1 mg of letrozole, a powerful nonsteroidal aromatase inhibitor, 1 hr before the steroid injections almost completely blocked the synaptic response to testosterone, resulting in a mean synapse density (0.723 +/- 0.003 spine synapse/microm3) only slightly higher than in OVX control rats. By contrast, the response to DHT was unaffected by letrozole pretreatment. These data suggest that androgen secretion during the female reproductive cycle may contribute to cyclical changes in hippocampal synaptic density. They also indicate that androgen treatment may be as effective as estrogen replacement in reversing the decline in hippocampal CA1 spine synapses that follows loss of ovarian function. Induction of hippocampal synapse formation by androgen is not mediated entirely via intracerebral estrogen biosynthesis, however, because aromatase-independent mechanisms also significantly affect CA1 spine synapse density.