Loss of aromatase cytochrome P450 function as a risk factor for Parkinson's disease?

The potential impact of CYP and UGT drug-metabolizing enzymes on brain target site drug exposure

Drug metabolism is one of the critical determinants of drug disposition throughout the body. While traditionally associated with the liver, recent research has unveiled the presence and functional significance of drug-metabolizing enzymes (DMEs) within the brain. Specifically, cytochrome P-450 enzymes (CYPs) and UDP-glucuronosyltransferases (UGTs) enzymes have emerged as key players in drug biotransformation within the central nervous system (CNS). This comprehensive review explores the cellular and subcellular distribution of CYPs and UGTs within the CNS, emphasizing regional expression and contrasting profiles between the liver and brain, humans and rats. Moreover, we discuss the impact of species and sex differences on CYPs and UGTs within the CNS. This review also provides an overview of methodologies for identifying and quantifying enzyme activities in the brain. Additionally, we present factors influencing CYPs and UGTs activities in the brain, including genetic polymorphisms, physiological variables, pathophysiological conditions, and environmental factors. Examples of CYP- and UGT-mediated drug metabolism within the brain are presented at the end, illustrating the pivotal role of these enzymes in drug therapy and potential toxicity. In conclusion, this review enhances our understanding of drug metabolism's significance in the brain, with a specific focus on CYPs and UGTs. Insights into the expression, activity, and influential factors of these enzymes within the CNS have crucial implications for drug development, the design of safe drug treatment strategies, and the comprehension of drug actions within the CNS. To that end, CNS pharmacokinetic (PK) models can be improved to further advance drug development and personalized therapy.

Subsecond Codetection of Dopamine and Estradiol at a Modified Sharkfin Waveform

17β-Estradiol (E2) is a ubiquitously expressed hormone that is active in a wide range of neuroprotective and regenerative roles throughout the brain. In particular, it is a well-known dopamine (DA) regulator and is responsible for modulating the expression of dopaminergic receptors and transporters. Recent studies point to E2 release occurring on a rapid time scale and having impacts on DA activity within seconds to minutes. As such, tools capable of monitoring the release of both E2 and DA in real time are essential for developing an accurate understanding of their interactive roles in neurotransmission and regulation. Currently, no analytical techniques capable of codetection of both analytes with high sensitivity, spatiotemporal resolution, extended monitoring, and minimal tissue damage exist. We describe a modified waveform using fast-scan cyclic voltammetry that is capable of low nanomolar detection of both DA and E2 on a subsecond time scale. Both analytes have limits of detection at or below 30 nM and high sensitivity: 11.31 ± 0.55 nA/μM for DA and 9.47 ± 0.36 nA/μM for E2. The waveform is validated in a tissue matrix, confirming its viability for measurement in a biologically relevant setting. This is the first method capable of codetection of fluctuations in DA and E2 with the temporal, spatial, and sensitivity requirements necessary for studying real-time neurochemical signaling.

Neuroprotective role for RORA in Parkinson's disease revealed by analysis of post-mortem brain and a dopaminergic cell line

Parkinson's disease (PD) is almost twice as prevalent in men, which has largely been attributed to neuroprotective effect of oestradiol in women. RORA (retinoic acid receptor-related orphan receptor alpha) regulates the transcription of central aromatase, the enzyme responsible for local oestradiol synthesis, simultaneously, RORA expression is regulated by sex hormones. Moreover, RORA protects neurones against oxidative stress, a key mechanism contributing to the loss of dopaminergic neurones in PD. Therefore, we hypothesized that there would be sex differences in RORA expression in the substantia nigra pars compacta (SNpc), which could contribute to sex differences observed in PD prevalence and pathogenesis. In a case control study, qPCR and western blot analyses were used to quantify gene and protein expression in the SNpc of post-mortem brains (n = 14 late-stage PD and 11 age and sex matched controls). The neuroprotective properties of a RORA agonist were then investigated directly using a cell culture toxin-based model of PD coupled with measures of viability, mitochondrial function and apoptosis. RORA was expressed at significantly higher levels in the SNpc from control females' brains compared to males. In PD, we found a significant increase in SNpc RORA expression in male PD compared to female PD. Treatment with a RORA agonist showed a significant neuroprotection in our cell culture model of PD and revealed significant effects on intracellular factors involved in neuronal survival and demise. This study is the first to demonstrate a sex specific pattern of RORA protein and gene expression in the SNpc of controls post-mortem human brains, and to show that this is differentially altered in male and female PD subjects, thus supporting a role for RORA in sex-specific aspects of PD. Furthermore, our in vitro PD model indicates mechanisms whereby a RORA agonist exerts its neuroprotective effect, thereby highlighting the translational potential for RORA ligands in PD.

Sex and Brain: The Role of Sex Chromosomes and Hormones in Brain Development and Parkinson's Disease

Sex hormones and genes on the sex chromosomes are not only key factors in the regulation of sexual differentiation and reproduction but they are also deeply involved in brain homeostasis. Their action is crucial for the development of the brain, which presents different characteristics depending on the sex of individuals. The role of these players in the brain is fundamental in the maintenance of brain function during adulthood as well, thus being important also with respect to age-related neurodegenerative diseases. In this review, we explore the role of biological sex in the development of the brain and analyze its impact on the predisposition toward and the progression of neurodegenerative diseases. In particular, we focus on Parkinson's disease, a neurodegenerative disorder that has a higher incidence in the male population. We report how sex hormones and genes encoded by the sex chromosomes could protect from the disease or alternatively predispose toward its development. We finally underline the importance of considering sex when studying brain physiology and pathology in cellular and animal models in order to better understand disease etiology and develop novel tailored therapeutic strategies.

Brain-Derived Estrogen and Neurological Disorders

Astrocytes and neurons in the male and female brains produce the neurosteroid brain-derived 17β-estradiol (BDE₂) from androgen precursors. In this review, we discuss evidence that suggest BDE₂ has a role in a number of neurological conditions, such as focal and global cerebral ischemia, traumatic brain injury, excitotoxicity, epilepsy, Alzheimer's disease, and Parkinson's disease. Much of what we have learned about BDE₂ in neurological disorders has come from use of aromatase inhibitors and global aromatase knockout mice. Recently, our group developed astrocyte- and neuron-specific aromatase knockout mice, which have helped to clarify the precise functions of astrocyte-derived 17β-estradiol (ADE₂) and neuron-derived 17β-estradiol (NDE₂) in the brain. The available evidence to date suggests a primarily beneficial role of BDE₂ in facilitating neuroprotection, synaptic and cognitive preservation, regulation of reactive astrocyte and microglia activation, and anti-inflammatory effects. Most of these beneficial effects appear to be due to ADE₂, which is induced in most neurological disorders, but there is also recent evidence that NDE₂ exerts similar beneficial effects. Furthermore, in certain situations, BDE₂ may also have deleterious effects, as recent evidence suggests its overproduction in epilepsy contributes to seizure induction. In this review, we examine the current state of this quickly developing topic, as well as possible future studies that may be required to provide continuing growth in the field.

Novel 18F-Labeled PET Tracers Specific to Aromatase: Design, Synthesis, and Biological Evaluation

The abnormal expression of aromatase is associated with the occurrence and development of a variety of neurological diseases and tumors. A series of ¹⁸F-labeled and ⁶⁸Ga-labeled potential aromatase-binding candidate compounds were designed and synthesized based on the structures of aromatase inhibitors. Competitive inhibition experiments in vitro and molecular docking showed that BIBD-069 and BIBD-071 have high affinity for aromatase. The radiolabeling conditions of [¹⁸F]BIBD-069 and [¹⁸F]BIBD-071 were simple, and the yields were high. Biodistribution and in vivo inhibition experiments confirmed that [¹⁸F]BIBD-069 and [¹⁸F]BIBD-071 specifically bind to aromatase. [¹⁸F]BIBD-069 and [¹⁸F]BIBD-071 selectively imaged the amygdala and nucleus of the stria terminalis, which is similar to the imaging result of [¹¹C]vorozole. Radiometabolites of [¹⁸F]BIBD-069 and [¹⁸F]BIBD-071 did not bind to aromatase and interfered with brain imaging. MicroPET-CT imaging further confirmed that [¹⁸F]BIBD-069 and [¹⁸F]BIBD-071 can specifically bind to aromatase and were not defluorinated in vivo. Given that [¹⁸F]BIBD-069 and [¹⁸F]BIBD-071 exhibit excellent aromatase binding affinities, mild radiolabeling conditions, and good pharmacokinetics, they can be important tools for the diagnosis and treatment of aromatase-related diseases.

Differential gene expression analysis following olfactory learning in honeybee (Apis mellifera L.)

Insects change their stimulus-response through the perception of associating these stimuli with important survival events such as rewards, threats, and mates. Insects develop strong associations and relate them to their experiences through several behavioral procedures. Among the insects, Apis species, Apis mellifera ligustica are known for their outstanding ability to learn with tremendous economic importance. Apis mellifera ligustica has a strong cognitive ability and promising model species for investigating the neurobiological basis of remarkable olfactory learning abilities. Here we evaluated the olfactory learning ability of A. mellifera by using the proboscis extension reflex (PER) protocol. The brains of the learner and failed-learner bees were examined for comparative transcriptome analysis by RNA-Seq to explain the difference in the learning capacity. In this study, we used an appetitive olfactory learning paradigm in the same age of A. mellifera bees to examine the differential gene expression in the brain of the learner and failed-learner. Bees that respond in 2nd and 3rd trials or only responded to 3rd trials were defined as learned bees, failed-learner individuals were those bees that did not respond in all learning trials The results indicate that the learning ability of learner bees was significantly higher than failed-learner bees for 12 days. We obtained approximately 46.7 and 46.4 million clean reads from the learner bees failed-learner bees, respectively. Gene expression profile between learners’ bees and failed-learners bees identified 74 differentially expressed genes, 57 genes up-regulated in the brains of learners and 17 genes were down-regulated in the brains of the bees that fail to learn. The qRT-PCR validated the differently expressed genes. Transcriptome analyses revealed that specific genes in learner and failed-learner bees either down-regulated or up-regulated play a crucial role in brain development and learning behavior. Our finding suggests that down-regulated genes of the brain involved in the integumentary system, storage proteins, brain development, sensory processing, and neurodegenerative disorder may result in reduced olfactory discrimination and olfactory sensitivity in failed-learner bees. This study aims to contribute to a better understanding of the olfactory learning behavior and gene expression information, which opens the door for understanding of the molecular mechanism of olfactory learning behavior in honeybees.

Estrogen receptors and the aging brain

The female sex hormone estrogen has been ascribed potent neuroprotective properties. It signals by binding and activating estrogen receptors that, depending on receptor subtype and upstream or downstream effectors, can mediate gene transcription and rapid non-genomic actions. In this way, estrogen receptors in the brain participate in modulating neural differentiation, proliferation, neuroinflammation, cholesterol metabolism, synaptic plasticity, and behavior. Circulating sex hormones decrease in the course of aging, more rapidly at menopause in women, and slower in men. This review will discuss what this drop entails in terms of modulating neuroprotection and resilience in the aging brain downstream of spatiotemporal estrogen receptor alpha (ERα) and beta (ERβ) signaling, as well as in terms of the sex differences observed in Alzheimer's disease (AD) and Parkinson's disease (PD). In addition, controversies related to ER expression in the brain will be discussed. Understanding the spatiotemporal signaling of sex hormones in the brain can lead to more personalized prevention strategies or therapies combating neurodegenerative diseases.

Wnt/β-Catenin Signaling Pathway Governs a Full Program for Dopaminergic Neuron Survival, Neurorescue and Regeneration in the MPTP Mouse Model of Parkinson's Disease

Wingless-type mouse mammary tumor virus (MMTV) integration site (Wnt) signaling is one of the most critical pathways in developing and adult tissues. In the brain, Wnt signaling contributes to different neurodevelopmental aspects ranging from differentiation to axonal extension, synapse formation, neurogenesis, and neuroprotection. Canonical Wnt signaling is mediated mainly by the multifunctional β-catenin protein which is a potent co-activator of transcription factors such as lymphoid enhancer factor (LEF) and T-cell factor (TCF). Accumulating evidence points to dysregulation of Wnt/β-catenin signaling in major neurodegenerative disorders. This review highlights a Wnt/β-catenin/glial connection in Parkinson's disease (PD), the most common movement disorder characterized by the selective death of midbrain dopaminergic (mDAergic) neuronal cell bodies in the subtantia nigra pars compacta (SNpc) and gliosis. Major findings of the last decade document that Wnt/β-catenin signaling in partnership with glial cells is critically involved in each step and at every level in the regulation of nigrostriatal DAergic neuronal health, protection, and regeneration in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD, focusing on Wnt/β-catenin signaling to boost a full neurorestorative program in PD.

The Toxicogenome of Hyalella azteca: A Model for Sediment Ecotoxicology and Evolutionary Toxicology

Hyalella azteca is a cryptic species complex of epibenthic amphipods of interest to ecotoxicology and evolutionary biology. It is the primary crustacean used in North America for sediment toxicity testing and an emerging model for molecular ecotoxicology. To provide molecular resources for sediment quality assessments and evolutionary studies, we sequenced, assembled, and annotated the genome of the H. azteca U.S. Lab Strain. The genome quality and completeness is comparable with other ecotoxicological model species. Through targeted investigation and use of gene expression data sets of H. azteca exposed to pesticides, metals, and other emerging contaminants, we annotated and characterized the major gene families involved in sequestration, detoxification, oxidative stress, and toxicant response. Our results revealed gene loss related to light sensing, but a large expansion in chemoreceptors, likely underlying sensory shifts necessary in their low light habitats. Gene family expansions were also noted for cytochrome P450 genes, cuticle proteins, ion transporters, and include recent gene duplications in the metal sequestration protein, metallothionein. Mapping of differentially expressed transcripts to the genome significantly increased the ability to functionally annotate toxicant responsive genes. The H. azteca genome will greatly facilitate development of genomic tools for environmental assessments and promote an understanding of how evolution shapes toxicological pathways with implications for environmental and human health.

The Toxicogenome of Hyalella azteca: A Model for Sediment Ecotoxicology and Evolutionary Toxicology

Hyalella azteca is a cryptic species complex of epibenthic amphipods of interest to ecotoxicology and evolutionary biology. It is the primary crustacean used in North America for sediment toxicity testing and an emerging model for molecular ecotoxicology. To provide molecular resources for sediment quality assessments and evolutionary studies, we sequenced, assembled, and annotated the genome of the H. azteca U.S. Lab Strain. The genome quality and completeness is comparable with other ecotoxicological model species. Through targeted investigation and use of gene expression data sets of H. azteca exposed to pesticides, metals, and other emerging contaminants, we annotated and characterized the major gene families involved in sequestration, detoxification, oxidative stress, and toxicant response. Our results revealed gene loss related to light sensing, but a large expansion in chemoreceptors, likely underlying sensory shifts necessary in their low light habitats. Gene family expansions were also noted for cytochrome P450 genes, cuticle proteins, ion transporters, and include recent gene duplications in the metal sequestration protein, metallothionein. Mapping of differentially expressed transcripts to the genome significantly increased the ability to functionally annotate toxicant responsive genes. The H. azteca genome will greatly facilitate development of genomic tools for environmental assessments and promote an understanding of how evolution shapes toxicological pathways with implications for environmental and human health.

Repurposing steroidogenesis inhibitors for the therapy of neuropsychiatric disorders: Promises and caveats

Steroids exert a profound influence on behavioral reactivity, by modulating the functions of most neurotransmitters and shaping the impact of stress and sex-related variables on neural processes. This background - as well as the observation that most neuroactive steroids (including sex hormones, glucocorticoids and neurosteroids) are synthetized and metabolized by overlapping enzymatic machineries - points to steroidogenic pathways as a powerful source of targets for neuropsychiatric disorders. Inhibitors of steroidogenic enzymes have been developed and approved for a broad range of genitourinary and endocrine dysfunctions, opening to new opportunities to repurpose these drugs for the treatment of mental problems. In line with this idea, preliminary clinical and preclinical results from our group have shown that inhibitors of key steroidogenic enzymes, such as 5α-reductase and 17,20 desmolase-lyase, may have therapeutic efficacy in specific behavioral disorders associated with dopaminergic hyperfunction. While the lack of specificity of these effects raises potential concerns about endocrine adverse events, these initial findings suggest that steroidogenesis modulators with greater brain specificity may hold significant potential for the development of alternative therapies for psychiatric problems. This article is part of the Special Issue entitled 'Drug Repurposing: old molecules, new ways to fast track drug discovery and development for CNS disorders'.

What does the research say about androgen use and cerebrovascular events?

Many studies have investigated the benefits of androgen therapy and neurosteroids in aging men, while concerns remain about the potential associations of exogenous steroids and incidents of cerebrovascular events and ischemic stroke (IS). Testosterone is neuroprotective, neurotrophic and a potent stimulator of neuroplasticity. These benefits are mediated primarily through conversion of a small amount of testosterone to estradiol by the catalytic activity of estrogen synthetase (aromatase cytochrome P450 enzyme). New studies suggest that abnormal serum levels of the nonaromatized potent metabolite of testosterone, either high or low dihydrotestosterone (DHT), is a risk factor for stroke. Associations between pharmacologic androgen use and the incidence of IS are questionable, because a significant portion of testosterone is converted to DHT. There is also insufficient evidence to reject a causal relationship between the pro-testosterone adrenal androgens and incidence of IS. Moreover, vascular intima-media thickness, which is a predictor of stroke and myocardial symptoms, has correlations with sex hormones. Current diagnostic and treatment criteria for androgen therapy for cerebrovascular complications are unclear. Confounding variables, including genetic and metabolic alterations of the key enzymes of steroidogenesis, ought to be considered. Information extracted from pharmacogenetic testing may aid in expounding the protective-destructive properties of neurosteroids, as well as the prognosis of androgen therapy, in particular their cerebrovascular outcomes. This investigative review article addresses relevant findings of the clinical and experimental investigations of androgen therapy, emphasizes the significance of genetic testing of androgen responsiveness towards individualized therapy in post-IS injuries as well as identifying pertinent questions.

Microglia Polarization, Gene-Environment Interactions and Wnt/β-Catenin Signaling: Emerging Roles of Glia-Neuron and Glia-Stem/Neuroprogenitor Crosstalk for Dopaminergic Neurorestoration in Aged Parkinsonian Brain

Neuroinflammatory processes are recognized key contributory factors in Parkinson's disease (PD) physiopathology. While the causes responsible for the progressive loss of midbrain dopaminergic (mDA) neuronal cell bodies in the subtantia nigra pars compacta are poorly understood, aging, genetics, environmental toxicity, and particularly inflammation, represent prominent etiological factors in PD development. Especially, reactive astrocytes, microglial cells, and infiltrating monocyte-derived macrophages play dual beneficial/harmful effects, via a panel of pro- or anti-inflammatory cytokines, chemokines, neurotrophic and neurogenic transcription factors. Notably, with age, microglia may adopt a potent neurotoxic, pro-inflammatory “primed” (M1) phenotype when challenged with inflammatory or neurotoxic stimuli that hamper brain's own restorative potential and inhibit endogenous neurorepair mechanisms. In the last decade we have provided evidence for a major role of microglial crosstalk with astrocytes, mDA neurons and neural stem progenitor cells (NSCs) in the MPTP- (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-) mouse model of PD, and identified Wnt/β-catenin signaling, a pivotal morphogen for mDA neurodevelopment, neuroprotection, and neuroinflammatory modulation, as a critical actor in glia-neuron and glia-NSCs crosstalk. With age however, Wnt signaling and glia-NSC-neuron crosstalk become dysfunctional with harmful consequences for mDA neuron plasticity and repair. These findings are of importance given the deregulation of Wnt signaling in PD and the emerging link between most PD related genes, Wnt signaling and inflammation. Especially, in light of the expanding field of microRNAs and inflammatory PD-related genes as modulators of microglial-proinflammatory status, uncovering the complex molecular circuitry linking PD and neuroinflammation will permit the identification of new druggable targets for the cure of the disease. Here we summarize recent findings unveiling major microglial inflammatory and oxidative stress pathways converging in the regulation of Wnt/β-catenin signaling, and reciprocally, the ability of Wnt signaling pathways to modulate microglial activation in PD. Unraveling the key factors and conditons promoting the switch of the proinflammatory M1 microglia status into a neuroprotective and regenerative M2 phenotype will have important consequences for neuroimmune interactions and neuronal outcome under inflammatory and/or neurodegenerative conditions.

Relation among Aromatase P450 and Tumoral Growth in Human Prolactinomas

The pituitary gland is part of hypothalamic-pituitary–gonadal axis, which controls development, reproduction, and aging in humans and animals. In addition, the pituitary gland is regulated mainly by hormones and neurotransmitters released from the hypothalamus and by systemic hormones secreted by target glands. Aromatase P450, the enzyme responsible for the catabolization of aromatizable androgens to estrogens, is expressed in different parts of body, including the pituitary gland. Moreover, aromatase P450 is involved in sexual dimorphism where alteration in the level of aromatase can initiate a number of diseases in both genders. On the other hand, the direct actions of estrogens, mainly estradiol, are well known for stimulating prolactin release. Numerous studies have shown that changes in the levels of estrogens, among other factors, have been implicated in the genesis and development of prolactinoma. The pituitary gland can produce estradiol locally in several types of endocrine cells, and it is possible that aromatase could be responsible for the maintenance of the population of lactotroph cells and the modulation of the action of central or peripheral regulators. Aromatase overexpression due to inappropriate gene regulation has clinical effects such as the pathogenesis of prolactinomas. The present study reports on the synthesis of pituitary aromatase, its regulation by gonadal steroids, and the physiological roles of aromatase on pituitary endocrine cells. The involvement of aromatase in the pathogenesis of pituitary tumors, mainly prolactinomas, through the auto-paracrine production of estradiol is reviewed.

Systematic analysis of microarray datasets to identify Parkinson's disease‑associated pathways and genes

In order to investigate commonly disturbed genes and pathways in various brain regions of patients with Parkinson's disease (PD), microarray datasets from previous studies were collected and systematically analyzed. Different normalization methods were applied to microarray datasets from different platforms. A strategy combining gene co-expression networks and clinical information was adopted, using weighted gene co-expression network analysis (WGCNA) to screen for commonly disturbed genes in different brain regions of patients with PD. Functional enrichment analysis of commonly disturbed genes was performed using the Database for Annotation, Visualization, and Integrated Discovery (DAVID). Co-pathway relationships were identified with Pearson's correlation coefficient tests and a hypergeometric distribution-based test. Common genes in pathway pairs were selected out and regarded as risk genes. A total of 17 microarray datasets from 7 platforms were retained for further analysis. Five gene coexpression modules were identified, containing 9,745, 736, 233, 101 and 93 genes, respectively. One module was significantly correlated with PD samples and thus the 736 genes it contained were considered to be candidate PD-associated genes. Functional enrichment analysis demonstrated that these genes were implicated in oxidative phosphorylation and PD. A total of 44 pathway pairs and 52 risk genes were revealed, and a risk gene pathway relationship network was constructed. Eight modules were identified and were revealed to be associated with PD, cancers and metabolism. A number of disturbed pathways and risk genes were unveiled in PD, and these findings may help advance understanding of PD pathogenesis.

Estrogen modulation of calretinin and BDNF expression in midbrain dopaminergic neurons of ovariectomised mice

Estrogen attenuates the loss of dopamine neurons from the substantia nigra in animal models of Parkinson's disease (PD) and excitatory amino-acid induced neurotoxicity by interactions with brain-derived neurotrophic factor (BDNF), and calretinin (CR) containing dopaminergic (DA) neurons. To examine this interaction more closely, we treated the ovariectomised (OVX) mice with estrodial for 10days, and compared these mice to those OVX mice injected with the vehicle or control mice. Estrogen treatment in OVX mice had significantly more tyrosine hydroxylase (TH) positive neurons in the substantia nigra pars compacta (SNpc). Dopamine transporter (DAT) mRNA and BDNF mRNA levels in the midbrain were also significantly increased by estrogen treatment (P<0.05). OVX markedly decreased the number of TH/CR double stained cells in the SNpc (P<0.05), a trend which could be reversed by estrogen treatment. However, the number of GFAP positive cells in the substantia nigra did not show significant changes (P >0.05) after vehicle or estrodial treatment. Furthermore, we found that estrogen treatment abrogated the OVX-induced decrease in the phosphorylated AKT (p-AKT), but not p-ERK. We hypothesize that short-term treatment with estrogen confers neuroprotection to DA neurons by increasing CR in the DA neurons and BDNF in the midbrain, which possibly related to activation of the PI3K/Akt signaling pathway.

Selective regulation of neurosteroid biosynthesis under ketamine-induced apoptosis of cortical neurons in vitro

Numerous studies have suggested that ketamine administration can induce neuroapoptosis in primary cultured cortical neurons. Neurosteroids modulate neuronal function and serve important roles in the central nervous system, however the role of neurosteroids in neuroapoptosis induced by ketamine remains to be elucidated. The present study aimed to explore whether neurosteroidogenesis was a pivotal mechanism for neuroprotection against ketamine-induced neuroapoptosis, and whether it may be selectively regulated under ketamine-induced neuroapoptosis conditions in primary cultured cortical neurons. To study this hypothesis, the effect of ketamine exposure on neurosteroidogenesis in primary cultured cortical neurons was investigated. Cholesterol, a substrate involved in the synthesis of neurosteroids, was added to the culture medium, and neurosteroids were quantified using high-performance liquid chromatography-tandem mass spectrometry analysis. The data demonstrated that cholesterol blocked ketamine-induced neuroapoptosis by promoting the synthesis of various neurosteroids, and the pathway of neurosteroid testosterone conversion into estradiol was inhibited by ketamine exposure. These data suggest that endogenous neurosteroids biosynthesis is critical for neuroprotection against ketamine-induced neuroapoptosis and inhibiting the biosynthesis of neuroprotective-neurosteroid estradiol is of notable importance for ketamine-induced neuroapoptosis.

Sex differences and effects of prenatal exposure to excess testosterone on ventral tegmental area dopamine neurons in adult sheep

Prenatal testosterone (T) excess in sheep results in a wide array of reproductive neuroendocrine deficits and alterations in motivated behavior. The ventral tegmental area (VTA) plays a critical role in reward and motivated behaviors and is hypothesised to be targeted by prenatal T. Here we report a sex difference in the number VTA dopamine cells in the adult sheep, with higher numbers of tyrosine hydroxylase (TH)-immunoreactive (-ir) cells in males than females. Moreover, prenatal exposure to excess T during either gestational days 30-90 or 60-90 resulted in increased numbers of VTA TH-ir cells in adult ewes compared to control females. Stereological analysis confirmed significantly greater numbers of neurons in the VTA of males and prenatal T-treated ewes, which was primarily accounted for by greater numbers of TH-ir cells. In addition, immunoreactivity for TH in the cells was denser in males and prenatal T-treated females, suggesting that sex differences and prenatal exposure to excess T affects both numbers of cells expressing TH and the protein levels within dopamine cells. Sex differences were also noted in numbers of TH-ir cells in the substantia nigra, with more cells in males than females. However, prenatal exposure to excess T did not affect numbers of TH-ir cells in the substantia nigra, suggesting that this sex difference is organised independently of prenatal actions of T. Together, these results demonstrate sex differences in the sheep VTA dopamine system which are mimicked by prenatal treatment with excess T.

Human aromatase: perspectives in biochemistry and biotechnology

Aromatase (CYP19) is involved in steroidogenesis, catalyzing the conversion of androgens into estrogens through a unique reaction that causes the aromatization of the A ring of the steroid. The enzyme is widely distributed and well conserved among species as it plays a crucial role in physiological processes such as control of reproduction and neuroprotection. It has also been a subject of intense research both at the biotechnological level in drug development due to its involvement in estrogen-dependent tumors and at a fundamental biochemical level because there are numerous questions regarding its reaction mechanism. This review will report the great progress made in this area.

11C-cetrozole: an improved C-11C-methylated PET probe for aromatase imaging in the brain

Aromatase (an enzyme that converts androgens to estrogens) in the brain is involved in neuroprotection, synaptic plasticity, and regulation of sexual and emotional behaviors. To investigate the physiologic and pathologic importance of aromatase in the brain, including in humans, we here report the development of a novel PET probe for aromatase, (11)C-cetrozole, which allows noninvasive quantification of aromatase expression.

METHODS

(11)C-cetrozole was synthesized by the C-(11)C-methylation method developed by our group. In vitro autoradiography of frozen sections and a binding study with rat brain homogenates were conducted to demonstrate the specific binding and the dissociation constant. PET studies with anesthetized rhesus monkeys were performed to analyze the dynamics in the brain.

RESULTS

In vitro and in vivo studies using (11)C-cetrozole showed its superiority in brain aromatase imaging in terms of specificity and selectivity, compared with previously developed (11)C-vorozole. PET studies showed that (11)C-cetrozole had a higher signal-to-noise ratio, providing a sharper image than (11)C-vorozole, because the radioactive metabolite of (11)C-vorozole was taken up into the brain. High specific binding of (11)C-cetrozole was observed in the amygdala and hypothalamus, and we also noted binding in the nucleus accumbens of rhesus monkeys for the first time.

CONCLUSION

These results suggest that PET imaging with newly developed (11)C-cetrozole is suitable for quantifying the expression of brain aromatase in vivo, possibly providing critical information regarding the functional roles of aromatase in human neurologic and emotional disorders.

Targeting Wnt signaling at the neuroimmune interface for dopaminergic neuroprotection/repair in Parkinson's disease

During the past three decades, the Wingless-type MMTV integration site (Wnt) signaling cascade has emerged as an essential system regulating multiple processes in developing and adult brain. Accumulating evidence points to a dysregulation of Wnt signaling in major neurodegenerative pathologies including Parkinson's disease (PD), a common neurodegenerative disorder characterized by the progressive loss of midbrain dopaminergic (mDA) neurons and deregulated activation of astrocytes and microglia. This review highlights the emerging link between Wnt signaling and key inflammatory pathways during mDA neuron damage/repair in PD progression. In particular, we summarize recent evidence documenting that aging and neurotoxicant exposure strongly antagonize Wnt/β-catenin signaling in mDA neurons and subventricular zone (SVZ) neuroprogenitors via astrocyte-microglial interactions. Dysregulation of the crosstalk between Wnt/β-catenin signaling and anti-oxidant/anti-inflammatory pathways delineate novel mechanisms driving the decline of SVZ plasticity with age and the limited nigrostriatal dopaminergic self-repair in PD. These findings hold a promise in developing therapies that target Wnt/β-catenin signaling to enhance endogenous restoration and neuronal outcome in age-dependent diseases, such as PD.

Neuroprotection by gonadal steroid hormones in acute brain damage requires cooperation with astroglia and microglia

The neuroactive steroids 17β-estradiol and progesterone control a broad spectrum of neural functions. Besides their roles in the regulation of classical neuroendocrine loops, they strongly influence motor and cognitive systems, behavior, and modulate brain performance at almost every level. Such a statement is underpinned by the widespread and lifelong expression pattern of all types of classical and non-classical estrogen and progesterone receptors in the CNS. The life-sustaining power of neurosteroids for tattered or seriously damaged neurons aroused interest in the scientific community in the past years to study their ability for therapeutic use under neuropathological challenges. Documented by excellent studies either performed in vitro or in adequate animal models mimicking acute toxic or chronic neurodegenerative brain disorders, both hormones revealed a high potency to protect neurons from damage and saved neural systems from collapse. Unfortunately, neurons, astroglia, microglia, and oligodendrocytes are comparably target cells for both steroid hormones. This hampers the precise assignment and understanding of neuroprotective cellular mechanisms activated by both steroids. In this article, we strive for a better comprehension of the mutual reaction between these steroid hormones and the two major glial cell types involved in the maintenance of brain homeostasis, astroglia and microglia, during acute traumatic brain injuries such as stroke and hypoxia. In particular, we attempt to summarize steroid-activated cellular signaling pathways and molecular responses in these cells and their contribution to dampening neuroinflammation and neural destruction. This article is part of a Special Issue entitled 'CSR 2013'.

Uncovering novel actors in astrocyte-neuron crosstalk in Parkinson's disease: the Wnt/β-catenin signaling cascade as the common final pathway for neuroprotection and self-repair

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by progressive loss of dopaminergic (DAergic) neuronal cell bodies in the substantia nigra pars compacta and gliosis. The cause and mechanisms underlying the demise of nigrostriatal DAergic neurons are ill-defined, but interactions between genes and environmental factors are recognized to play a critical role in modulating the vulnerability to PD. Current evidence points to reactive glia as a pivotal factor in PD pathophysiology, playing both protective and destructive roles. Here, the contribution of reactive astrocytes and their ability to modulate DAergic neurodegeneration, neuroprotection and neurorepair in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) rodent model of PD will be discussed in the light of novel emerging evidence implicating wingless-type mouse mammary tumor virus integration site (Wnt)/β-catenin signaling as a strong candidate in MPTP-induced nigrostriatal DAergic plasticity. In this work, we highlight an intrinsic Wnt1/frizzled-1/β-catenin tone that critically contributes to the survival and protection of adult midbrain DAergic neurons, with potential implications for drug design or drug action in PD. The dynamic interplay between astrocyte-derived factors and neurogenic signals in MPTP-induced nigrostriatal DAergic neurotoxicity and repair will be summarized, together with recent findings showing a critical role of glia-neural stem/progenitor cell (NPC) interactions aimed at overcoming neurodegeneration and inducing neurorestoration. Understanding the intrinsic plasticity of nigrostriatal DAergic neurons and deciphering the signals facilitating the crosstalk between astrocytes, microglia, DAergic neurons and NPCs may have major implications for the role of stem cell technology in PD, and for identifying potential therapeutic targets to induce endogenous neurorepair.

Aging-induced Nrf2-ARE pathway disruption in the subventricular zone drives neurogenic impairment in parkinsonian mice via PI3K-Wnt/β-catenin dysregulation

Aging and exposure to environmental toxins including MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) are strong risk factors for developing Parkinson's disease (PD), a common neurologic disorder characterized by selective degeneration of midbrain dopaminergic (DAergic) neurons and astrogliosis. Aging and PD impair the subventricular zone (SVZ), one of the most important brain regions for adult neurogenesis. Because inflammation and oxidative stress are the hallmarks of aging and PD, we investigated the nature, timing, and signaling mechanisms contributing to aging-induced SVZ stem/neuroprogenitor cell (NPC) inhibition in aging male mice and attempted to determine to what extent manipulation of these pathways produces a functional response in the outcome of MPTP-induced DAergic toxicity. We herein reveal an imbalance of Nrf2-driven antioxidant/anti-inflammatory genes, such as Heme oxygenase1 in the SVZ niche, starting by middle age, amplified upon neurotoxin treatment and associated with an exacerbated proinflammatory SVZ microenvironment converging to dysregulate the Wingless-type MMTV integration site (Wnt)/β-catenin signaling, a key regulatory pathway for adult NPCs. In vitro experiments using coculture paradigms uncovered aged microglial proinflammatory mediators as critical inhibitors of NPC proliferative potential. We also found that interruption of PI3K (phosphatidylinositol3-kinase)/Akt and the Wnt/Fzd/β-catenin signaling cascades, which switch glycogen synthase kinase 3β (GSK-3β) activation on and off, were causally related to the impairment of SVZ-NPCs. Moreover, a synergy between dysfunctional microglia of aging mice and MPTP exposure further inhibited astrocyte proneurogenic properties, including the expression of key Wnts components. Last, pharmacological activation/antagonism studies in vivo and in vitro suggest the potential that aged SVZ manipulation is associated with DAergic functional recovery.

Signaling pathways mediating the neuroprotective effects of sex steroids and SERMs in Parkinson's disease

Studies with the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) animal model of Parkinson's disease have shown the ability of 17β-estradiol to protect the nigrostriatal dopaminergic system. This paper reviews the signaling pathways mediating the neuroprotective effect of 17β-estradiol against MPTP-induced toxicity. The mechanisms of 17β-estradiol action implicate activation of signaling pathways such as the phosphatidylinositol-3 kinase/Akt and the mitogen-activated protein kinase pathways. 17β-estradiol signaling is complex and integrates multiple interactions with signaling molecules that act to potentiate a protective effect. 17β-estradiol signaling is mediated via estrogen receptors, including GPER1, but others receptors, such as the IGF-1 receptor, are implicated in the neuroprotective effect. Glial and neuronal crosstalk is a critical factor in the maintenance of dopamine neuronal survival and in the neuroprotective action of 17β-estradiol. Compounds that stimulate GPER1 such as selective estrogen receptor modulators and phytoestrogens show neuroprotective activity and are alternatives to 17β-estradiol.

Cytochrome P450 enzymes in the brain: emerging evidence of biological significance

Cytochrome P450 (CYP) enzymes are responsible for the metabolism of many exogenous and endogenous compounds. CYPs are abundant in the liver and are also expressed in many extra-hepatic tissues including the brain. Although total CYP levels in the brain are much lower than in the liver, brain CYPs are concentrated near drug targets in specific regions and cell types, and can potentially have a considerable impact on local metabolism. Individual differences in brain CYP metabolism, due to inducers, inhibitors or genetic variation, can influence sensitivity and response to centrally acting drugs. Brain CYPs may also play a role in modulating brain activity, behavior, susceptibility to central nervous system diseases and treatment outcomes. This review highlights recent progress that has been made in understanding the functional significance of CYPs in the brain.

Aromatase and its inhibition in behaviour, obsessive compulsive disorder and parkinsonism

Oestrogens regulate normal behaviour and have been implicated in modulating pathological behaviour such as obsessive compulsive disorder and neurological disorder such as Parkinsonism. Therefore, by regulating the expression of the oestrogen-synthesising enzyme, aromatase, we may identify what behaviour is regulated by oestrogen. Inhibition of aromatase either genetically or pharmacologically has been reported to induce sexual behaviour impairment, compulsive behaviour and susceptibility to neurodegeneration.

Gender and manganese exposure interactions on mouse striatal neuron morphology

Gender differences in sensitivity and toxicokinetics of multiple metals have been identified in humans. A recent study suggested that young girls performed worse on intellectual exams than young boys exposed to manganese (Mn) in the environment. Animal studies have shown that Mn exposure causes differential effects on behavior in male compared to female mice. We hypothesized that in response to Mn exposure striatal Mn accumulation and/or striatal medium spiny neuron (MSN) morphology show gender-dependent effects. We evaluated the contribution of gender to neuropathology by examining striatal MSN morphology in male and female mice exposed to Mn. We found that gender played a significant role in alterations of striatal MSN morphology in mice exposed to Mn. Gender-dependent changes were strongest when striatal Mn levels were elevated 24h following the final Mn exposure. Nevertheless, gender-dependent alterations in neuron morphology were still present 3 weeks after the final Mn exposure. Gender differences in neuron morphology were not due to differential striatal Mn accumulation between genders. We conclude that although gender does not affect striatal Mn accumulation, MSN morphology is differentially sensitive to elevated Mn levels.

Intracerebral estrogen provision increases cytogenesis and neurogenesis in the injured zebra finch brain

To determine whether or not local, injury-induced aromatization and/or estrogen provision can affect cyto- or neuro-genesis following mechanical brain damage, two groups of adult male zebra finches sustained bilateral penetrating brain injuries. The first received contralateral injections of vehicle or the aromatase inhibitor fadrozole. The second group received contalateral injections of fadrozole, or fadrozole with 17β-estradiol. Subsequent to injury, birds were injected with the thymidine analog 5-bromo-2'-deoxyuridine (BrdU). Two weeks following injury, the birds were perfused, and coronal sections were labeled using antibodies against BrdU and the neuronal proteins HuC/HuD. In a double blind fashion, BrdU positive cells and BrdU/Hu double-labeled cells in the subventricular zone (SVZ) and at the injury site (INJ) were imaged and sampled. The average numbers of cells per image were compared across brain regions and treatments using repeated measures ANOVAs and, where applicable, post-hoc, pairwise comparisons. Fadrozole administration had no detectable effect on cytogenesis or neurogenesis, however, fadrozole coupled with estradiol significantly increased both measures. The dorsal SVZ had the greatest proportion of new cells that differentiated into neurons, though the highest numbers of BrdU labeled and BrdU, Hu double-labeled cells were detected at the INJ. In the adult zebra finch brain, local estradiol provision can increase cytogenesis and neurogenesis, however, whether or not endogenous glial aromatization is sufficient to similarly affect these processes remains to be seen.

Sex differences in the injured brain

Observations obtained in human and in experimental models clearly demonstrate sex differences in degenerative events occurring in the central nervous system. The present review focuses on potential factors that may contribute to these sex-dimorphic features; in particular, morphological organization of the central nervous system and functional influence by neuroactive steroids, genes, and immune system are considered.

Male/Female differences in neuroprotection and neuromodulation of brain dopamine

The existence of a sex difference in Parkinson's disease (PD) is observed as related to several variables, including susceptibility of the disease, age at onset, and symptoms. These differences between men and women represent a significant characteristic of PD, which suggest that estrogens may exert beneficial effects against the development and the progression of the disease. This paper reviews the neuroprotective and neuromodulator effects of 17β-estradiol and progesterone as compared to androgens in the nigrostriatal dopaminergic (NSDA) system of both female and male rodents. The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mice model of PD and methamphetamine toxicity faithfully reproduce the sex differences of PD in that endogenous estrogen levels appear to influence the vulnerability to toxins targeting the NSDA system. Exogenous 17β-estradiol and/or progesterone treatments show neuroprotective properties against NSDA toxins while androgens fail to induce any beneficial effect. Sex steroid treatments show male and female differences in their neuroprotective action against methamphetamine toxicity. NSDA structure and function, as well as the distribution of estrogen receptors, show sex differences and may influence the susceptibility to the toxins and the response to sex steroids. Genomic and non-genomic actions of 17β-estradiol converge to promote survival factors and the presence of both estrogen receptors α and β are critical to 17β-estradiol neuroprotective action against MPTP toxicity.

Peripheral vs. Central Sex Steroid Hormones in Experimental Parkinson's Disease

The nigrostriatal dopaminergic (NSDA) pathway degenerates in Parkinson's disease (PD), which occurs with approximately twice the incidence in men than women. Studies of the influence of systemic estrogens in females suggest sex hormones contribute to these differences. In this review we analyze the evidence revealing great complexity in the response of the healthy and injured NSDA system to hormonal influences, and emphasize the importance of centrally generated estrogens. At physiological levels, circulating estrogen (in females) or estrogen precursors (testosterone in males, aromatized to estrogen centrally) have negligible effects on dopaminergic neuron survival in experimental PD, but can modify striatal dopamine levels via actions on the activity or adaptive responses of surviving cells. However, these effects are sexually dimorphic. In females, estradiol promotes adaptive responses in the partially injured NSDA pathway, preserving striatal dopamine, whereas in males gonadal steroids and exogenous estradiol have a negligible or even suppressive effect, effectively exacerbating dopamine loss. On balance, the different effects of gonadal factors in males and females contribute to sex differences in experimental PD. Fundamental sex differences in brain organization, including the sexually dimorphic networks regulating NSDA activity are likely to underpin these responses. In contrast, estrogen generated locally appears to preserve striatal dopamine in both sexes. The available data therefore highlight the need to understand the biological basis of sex-specific responses of the NSDA system to peripheral hormones, so as to realize the potential for sex-specific, hormone-based therapies in PD. Furthermore, they suggest that targeting central steroid generation could be equally effective in preserving striatal dopamine in both sexes. Clarification of the relative roles of peripheral and central sex steroid hormones is thus an important challenge for future studies.

Combining nitric oxide release with anti-inflammatory activity preserves nigrostriatal dopaminergic innervation and prevents motor impairment in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of Parkinson's disease

Background

Current evidence suggests a role of neuroinflammation in the pathogenesis of Parkinson's disease (PD) and in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of basal ganglia injury. Reportedly, nonsteroidal anti-inflammatory drugs (NSAIDs) mitigate DAergic neurotoxicity in rodent models of PD. Consistent with these findings, epidemiological analysis indicated that certain NSAIDs may prevent or delay the progression of PD. However, a serious impediment of chronic NSAID therapy, particularly in the elderly, is gastric, renal and cardiac toxicity. Nitric oxide (NO)-donating NSAIDs, have a safer profile while maintaining anti-inflammatory activity of parent compounds. We have investigated the oral activity of the NO-donating derivative of flurbiprofen, [2-fluoro-α-methyl (1,1'-biphenyl)-4-acetic-4-(nitrooxy)butyl ester], HCT1026 (30 mg kg^-1 daily in rodent chow) in mice exposed to the parkinsonian neurotoxin MPTP.

Methods

Ageing mice were fed with a control, flurbiprofen, or HCT1026 diet starting ten days before MPTP administration and continuing for all the experimental period. Striatal high affinity synaptosomial dopamine up-take, motor coordination assessed with the rotarod, tyrosine hydroxylase (TH)- and dopamine transporter (DAT) fiber staining, stereological cell counts, immunoblotting and gene expression analyses were used to assess MPTP-induced nigrostriatal DAergic toxicity and glial activation 1-40 days post-MPTP.

Results

HCT1026 was well tolerated and did not cause any measurable toxic effect, whereas flurbiprofen fed mice showed severe gastrointestinal side-effects. HCT1026 efficiently counteracted motor impairment and reversed MPTP-induced decreased synaptosomal [³H]dopamine uptake, TH- and DAT-stained fibers in striatum and TH⁺ neuron loss in subtantia nigra pars compacta (SNpc), as opposed to age-matched mice fed with a control diet. These effects were associated to a significant decrease in reactive macrophage antigen-1 (Mac-1)-positive microglial cells within the striatum and ventral midbrain, decreased expression of iNOS, Mac-1 and NADPH oxidase (PHOX), and downregulation of 3-Nitrotyrosine, a peroxynitrite finger print, in SNpc DAergic neurons.

Conclusions

Oral treatment with HCT1026 has a safe profile and a significant efficacy in counteracting MPTP-induced dopaminergic (DAergic) neurotoxicity, motor impairment and microglia activation in ageing mice. HCT1026 provides a novel promising approach towards the development of effective pharmacological neuroprotective strategies against PD.

Estrogen actions in the brain and the basis for differential action in men and women: a case for sex-specific medicines

The classic view of estrogen actions in the brain was confined to regulation of ovulation and reproductive behavior in the female of all mammalian species studied, including humans. Burgeoning evidence now documents profound effects of estrogens on learning, memory, and mood as well as neurodevelopmental and neurodegenerative processes. Most data derive from studies in females, but there is mounting recognition that estrogens play important roles in the male brain, where they can be generated from circulating testosterone by local aromatase enzymes or synthesized de novo by neurons and glia. Estrogen-based therapy therefore holds considerable promise for brain disorders that affect both men and women. However, as investigations are beginning to consider the role of estrogens in the male brain more carefully, it emerges that they have different, even opposite, effects as well as similar effects in male and female brains. This review focuses on these differences, including sex dimorphisms in the ability of estradiol to influence synaptic plasticity, neurotransmission, neurodegeneration, and cognition, which, we argue, are due in a large part to sex differences in the organization of the underlying circuitry. There are notable sex differences in the incidence and manifestations of virtually all central nervous system disorders, including neurodegenerative disease (Parkinson's and Alzheimer's), drug abuse, anxiety, and depression. Understanding the cellular and molecular basis of sex differences in brain physiology and responses to estrogen and estrogen mimics is, therefore, vitally important for understanding the nature and origins of sex-specific pathological conditions and for designing novel hormone-based therapeutic agents that will have optimal effectiveness in men or women.

Aromatase in the brain of teleost fish: expression, regulation and putative functions

Unlike that of mammals, the brain of teleost fish exhibits an intense aromatase activity due to the strong expression of one of two aromatase genes (aromatase A or cyp19a1a and aromatase B or cyp19a1b) that arose from a gene duplication event. In situ hybridization, immunohistochemistry and expression of GFP (green fluorescent protein) in transgenic tg(cyp19a1b-GFP) fish demonstrate that aromatase B is only expressed in radial glial cells (RGC) of adult fish. These cells persist throughout life and act as progenitors in the brain of both developing and adult fish. Although aromatase B-positive radial glial cells are most abundant in the preoptic area and the hypothalamus, they are observed throughout the entire central nervous system and spinal cord. In agreement with the fact that brain aromatase activity is correlated to sex steroid levels, the high expression of cyp19a1b is due to an auto-regulatory loop through which estrogens and aromatizable androgens up-regulate aromatase expression. This mechanism involves estrogen receptor binding on an estrogen response element located on the cyp19a1b promoter. Cell specificity is achieved by a mandatory cooperation between estrogen receptors and unidentified glial factors. Given the emerging roles of estrogens in neurogenesis, the unique feature of the adult fish brain suggests that, in addition to classical functions on brain sexual differentiation and sexual behaviour, aromatase expression in radial glial cells could be part of the mechanisms authorizing the maintenance of a high proliferative activity in the brain of fish.

Androgen decreases dopamine neurone survival in rat midbrain

Clinical studies show that men are more likely to develop disorders affecting midbrain dopaminergic pathways, such as drug addiction and Parkinson's disease (PD). Although a great deal of focus has been given to the role of oestrogen in the maintenance of midbrain dopaminergic pathways, little is known about how testosterone influences these pathways. In the present study, we used stereological analysis of tyrosine hydroxylase-immunoreactive (TH-IR) cell bodies to determine how testosterone influences the dopaminergic cell bodies of the substantia nigra pars compacta (SNpc) and ventral tegmental area (VTA). Rats and mice were castrated at postnatal day (PN) 60, and these midbrain cell populations were counted on PN 90. One month after castration, TH-IR cell number had increased in the SNpc and VTA of rats and mice. Replacement with testosterone or the non-aromatisable analogue dihydrotestosterone (DHT) in castrated animals reduced TH-IR cell number in the SNpc and VTA in rats. In mice, the decrease of TH-IR cell number with testosterone or DHT replacement was observed only in the SNpc. The apparent increase in TH-IR neurone number after castration is not explained by an increase in TH expression because the number of nondopaminergic cells (TH-immunonegative, TH-IN) did not decrease proportionally after castration. TH-IN cell number did not change after castration or hormone replacement in rat or mouse SNpc or VTA. These findings suggest that testosterone may play a suppressive role in midbrain dopaminergic pathways.

Therapeutic implications of brain steroidogenesis

The nervous system is a steroidogenic tissue and several steroids synthesized locally in the brain, such as pregnenolone, progesterone and estradiol, modulate neuronal and glial physiology and are neuroprotective. The brain upregulates steroidogenesis at sites of injury as part of a program triggered by neural tissue to cope with neurodegenerative insults. Pharmacological targets to increase brain steroidogenesis and promote neuroprotection include the molecules that transport cholesterol to the inner mitochondrial membrane, where the first enzyme for steroidogenesis is located. Furthermore, the human gene encoding aromatase, the enzyme that synthesizes estradiol, is under the control of different tissue-specific promoters, and it is therefore conceivable that selective aromatase modulators can be developed that will enhance the expression of the enzyme and the consequent increase in estrogen formation in the brain but not in other tissues.

Sex-specific therapeutic strategies based on neuroactive steroids: In search for innovative tools for neuroprotection

Different pathologies of the central and peripheral nervous system show sex differences in their incidence, symptomatology and/or neurodegenerative outcome. These include Parkinson's disease, Alzheimer's disease, Huntington's disease, multiple sclerosis, traumatic brain injury, stroke, autism, schizophrenia, depression, anxiety disorders, eating disorders and peripheral neuropathy. These sex differences reveal the need for sex-specific neuroprotective strategies. This review article and other manuscripts published in this issue of Hormones and Behavior analyze possible sex-specific therapeutic strategies based on neuroactive steroids. In particular in our introductory article, the possibility that sex differences in the levels or in the action of neuroactive steroids may represent causative factors for sex differences in the incidence or manifestation of pathologies of the nervous system is considered.

The multiple roles of estrogens and the enzyme aromatase

Aromatase is the enzyme that catalyzes the last step of estrogen biosynthesis. It is expressed in many tissues such as the gonads, brain and adipose tissue. The regulation of the level and activity of aromatase determines the levels of estrogens that have endocrine, paracrine and autocrine effects on tissues. Estrogens play many roles in the body, regulating reproduction, metabolism and behavior. In the brain, cell survival and the activity of neurons are affected by estrogens and hence aromatase.

Independent influences of sex steroids of systemic and central origin in a rat model of Parkinson's disease: A contribution to sex-specific neuroprotection by estrogens

This review considers evidence which reveals considerable complexity and sex differences in the response of the nigrostriatal dopaminergic (NSDA) system to hormonal influences. This pathway degenerates in Parkinson's disease (PD) and sex hormones contribute to sex differences in PD, where men fare worse than women. Here we discuss evidence from animal studies which allows us to hypothesize that, contrary to expectations, the acclaimed neuroprotective property of physiological concentrations of estradiol arises not by promoting NSDA neuron survival, but by targeting powerful adaptive responses in the surviving neurons, which restore striatal DA functionality until over 60% of neurons are lost. Estrogen generated locally in the NSDA region appears to promote these adaptive mechanisms in females and males to preserve striatal DA levels in the partially injured NSDA pathway. However, responses to systemic steroids differ between the sexes. In females there is general agreement that gonadal steroids and exogenous estradiol promote striatal adaptation in the partially injured NSDA pathway to protect against striatal DA loss. In contrast, the balance of evidence suggests that in males gonadal factors and exogenous estradiol have negligible or even harmful effects. Sex differences in the organization of NSDA-related circuitry may well account for these differences. Compensatory mechanisms and sexually dimorphic hard-wiring are therefore likely to represent important biological substrates for sex dimorphisms. As these processes may be targeted differentially by systemic steroids in males and females, further understanding of the underlying processes would provide valuable insights into the potential for hormone-based therapies in PD, which would need to be sex-specific. Alternatively, evidence that estrogen generated locally is protective in the injured male NSDA pathway indicates the great therapeutic potential of harnessing central steroid synthesis to ameliorate neurodegenerative disorders. A clearer understanding of the relative contributions and inter-relationships of central and systemic steroids within the NSDA system is an important goal for future studies.

Neuroprotective actions of brain aromatase

The steroidal regulation of vertebrate neuroanatomy and neurophysiology includes a seemingly unending list of brain areas, cellular structures and behaviors modulated by these hormones. Estrogens, in particular have emerged as potent neuromodulators, exerting a range of effects including neuroprotection and perhaps neural repair. In songbirds and mammals, the brain itself appears to be the site of injury-induced estrogen synthesis via the rapid transcription and translation of aromatase (estrogen synthase) in astroglia. This induction seems to occur regardless of the nature and location of primary brain damage. The induced expression of aromatase apparently elevates local estrogen levels enough to interfere with apoptotic pathways, thereby decreasing secondary degeneration and ultimately lessening the extent of damage. There is even evidence suggesting that aromatization may affect injury-induced cytogenesis. Thus, aromatization in the brain appears to confer neuroprotection by an array of mechanisms that involve the deceleration and acceleration of degeneration and repair, respectively. We are only beginning to understand the factors responsible for the injury-induced transcription of aromatase in astroglia. In contrast, much of the manner in which local and circulating estrogens may achieve their neuroprotective effects has been elucidated. However, gaps in our knowledge include issues about the cell-specific regulation of aromatase expression, steroidal influences of aromatization distinct from estrogen formation, and questions about the role of constitutive aromatase in neuroprotection. Here we describe the considerable consensus and some interesting differences in knowledge gained from studies conducted on diverse animal models, experimental paradigms and preparations towards understanding the neuroprotective actions of brain aromatase.

The endocrine nervous system: source and target for neuroactive steroids

For a long time the endocrine brain was considered to the hypothalamus and to its special relationships with the hypophysis. The discovery of the wide distribution of steroid hormone receptors, as well as that of the possibility of metabolizing or synthesizing steroids by neural cells (neuroactive steroids), suggest, on the contrary, that interactions among steroids and nervous system are key points of the regulatory processes in the central and peripheral nervous system in normal conditions as well as in pathological conditions. In this brief overview we illustrate a few examples of these relationships with major emphasis on papers collected in this special issue.

Aromatase in the brain: not just for reproduction anymore

Aromatase, the enzyme that synthesises oestrogens from androgen precursors, is expressed in the brain, where it has been classically associated with the regulation of neuroendocrine events and behaviours linked with reproduction. Recent findings, however, have revealed new unexpected roles for brain aromatase, indicating that the enzyme regulates synaptic activity, synaptic plasticity, neurogenesis and the response of neural tissue to injury, and may contribute to control nonreproductive behaviours, mood and cognition. Therefore, the function of brain aromatase is not restricted to the regulation of reproduction as previously thought.