Gonadal steroids as promoters of neuro-glial plasticity

Glial Cell Modulation of Dendritic Spine Structure and Synaptic Function

Glia comprise a heterogeneous group of cells involved in the structure and function of the central and peripheral nervous system. Glial cells are found from invertebrates to humans with morphological specializations related to the neural circuits in which they are embedded. Glial cells modulate neuronal functions, brain wiring and myelination, and information processing. For example, astrocytes send processes to the synaptic cleft, actively participate in the metabolism of neurotransmitters, and release gliotransmitters, whose multiple effects depend on the targeting cells. Human astrocytes are larger and more complex than their mice and rats counterparts. Astrocytes and microglia participate in the development and plasticity of neural circuits by modulating dendritic spines. Spines enhance neuronal connectivity, integrate most postsynaptic excitatory potentials, and balance the strength of each input. Not all central synapses are engulfed by astrocytic processes. When that relationship occurs, a different pattern for thin and large spines reflects an activity-dependent remodeling of motile astrocytic processes around presynaptic and postsynaptic elements. Microglia are equally relevant for synaptic processing, and both glial cells modulate the switch of neuroendocrine secretion and behavioral display needed for reproduction. In this chapter, we provide an overview of the structure, function, and plasticity of glial cells and relate them to synaptic maturation and modulation, also involving neurotrophic factors. Together, neurons and glia coordinate synaptic transmission in both normal and abnormal conditions. Neglected over decades, this exciting research field can unravel the complexity of species-specific neural cytoarchitecture as well as the dynamic region-specific functional interactions between diverse neurons and glial subtypes.

Changes in dendritic arborization related to the estrous cycle in pyramidal neurons of layer V of the motor cortex

Many studies on neuronal plasticity have been conducted in the hippocampus and sensory cortices. In female rats in the estrus phase, when there is a low concentration of estradiol in the blood, there is a reduction in the dendritic spine density of CA1 neurons, while an increase in dendritic spines has been observed during metestrus, when progesterone levels are high. In comparison with the hippocampus, less information is known about dendritic remodeling of the motor cortex. Thus, the objective of the present study was to evaluate the neuronal morphology of pyramidal cells of layer V of the motor cortex in each phase of the estrous cycle. For this, we used Long-Evans strain rats and formed 4 experimental groups according to the phase of the estrous cycle at the moment of sacrifice: proestrus, estrus, metestrus, or diestrus. All animals were gently monitored regarding the expression of one estrous cycle in order to determine the regularity of the cycle. We obtained the brains in order to evaluate the neuronal morphology of neurons of layer V of the primary motor cortex following the Golgi-Cox method and Sholl analysis. Our results show that the dendritic arborization of neurons of rats sacrificed in the metestrus phase is reduced compared to the other phases of the estrous cycle. However, we did not find changes in dendritic spine density between experimental groups. When comparing our results with previous data, we can suggest that estrogens and progesterone differentially promote plasticity events in pyramidal neurons between different brain regions.

Mitochondria, Estrogen and Female Brain Aging

Mitochondria play an essential role in the generation of steroid hormones including the female sex hormones. These hormones are, in turn, able to modulate mitochondrial activities. Mitochondria possess crucial roles in cell maintenance, survival and well-being, because they are the main source of energy as well as of reactive oxygen species (ROS) within the cell. The impairment of these important organelles is one of the central features of aging. In women’s health, estrogen plays an important role during adulthood not only in the estrous cycle, but also in the brain via neuroprotective, neurotrophic and antioxidant modes of action. The hypestrogenic state in the peri- as well as in the prolonged postmenopause might increase the vulnerability of elderly women to brain degeneration and age-related pathologies. However, the underlying mechanisms that affect these processes are not well elucidated. Understanding the relationship between estrogen and mitochondria might therefore provide better insights into the female aging process. Thus, in this review, we first describe mitochondrial dysfunction in the aging brain. Second, we discuss the estrogen-dependent actions on the mitochondrial activity, including recent evidence of the estrogen—brain-derived neurotrophic factor and estrogen—sirtuin 3 (SIRT3) pathways, as well as their potential implications during female aging.

Interactions between inflammation, sex steroids, and Alzheimer's disease risk factors

Alzheimer's disease (AD) is an age-related neurodegenerative disorder for which there are no effective strategies to prevent or slow its progression. Because AD is multifactorial, recent research has focused on understanding interactions among the numerous risk factors and mechanisms underlying the disease. One mechanism through which several risk factors may be acting is inflammation. AD is characterized by chronic inflammation that is observed before clinical onset of dementia. Several genetic and environmental risk factors for AD increase inflammation, including apolipoprotein E4, obesity, and air pollution. Additionally, sex steroid hormones appear to contribute to AD risk, with age-related losses of estrogens in women and androgens in men associated with increased risk. Importantly, sex steroid hormones have anti-inflammatory actions and can interact with several other AD risk factors. This review examines the individual and interactive roles of inflammation and sex steroid hormones in AD, as well as their relationships with the AD risk factors apolipoprotein E4, obesity, and air pollution.

Estrogen-Induced Hypothalamic Synaptic Plasticity and Pituitary Sensitization in the Control of the Estrogen-Induced Gonadotrophin Surge

Proper gonadal function requires coordinated (feedback) interactions between the gonads, adenohypophysis, and brain: the gonads elaborate sex steroids (progestins, androgens, and estrogens) and proteins (inhibin-activin family) during gamete development. In both sexes, the brain-pituitary gonadotrophin-regulating interaction is coordinated by estradiol through its opposing actions on pituitary gonadotrophs (sensitization of the response to gonadotrophin-releasing hormone [GnRH]) versus hypothalamic neurons (inhibition of GnRH secretion). This dynamic tension between the gonadotrophs and the GnRH cells in the brain regulates the circulating gonadotrophins and is termed reciprocal/negative feedback. In females, reciprocal/negative feedback dominates approximately 90% of the ovarian cycle. In a spectacular exception, the dynamic tension is broken during the surge of circulating estrogen that marks follicle and oocyte(s) maturation. The cause is an estradiol-induced disinhibition of the GnRH neurons that releases GnRH secretion to the highly sensitized pituitary gonadotrophs that in turn release the gonadotrophin surge (the estrogen-induced gonadotrophin surge [EIGS], also known as positive feedback). Studies during the past 4 decades have shown this disinhibition to result from estrogen-induced synaptic plasticity (EISP), including a reversible approximately 50% loss in arcuate nucleus synapses. The disinhibited GnRH secretion occurs during maximal gonadotroph sensitization and results in the EIGS. Specific immunoneutralization of estradiol blocks the EISP and EIGS. The EISP is accompanied by increases in insulinlike growth factor 1, polysialylated neural cell adhesion molecule, and ezrin, 3 proteins that the authors believe are the links between estrogen-induced astroglial extension and the EISP that releases GnRH secretion at the moment of maximal sensitization of the pituitary gonadotrophs. The result is the paradoxical surge of gonadotrophins at the peak of ovarian estrogen secretion and the triggering of ovulation. This enhanced understanding of the mechanics of gonadotrophin control clarifies elements of the involved feedback loops and opens the way to a better understanding of the neurobiology of reproduction.

Review : Progesterone and Recovery after Traumatic Brain Injury: An Overview

After traumatic brain injury (TBI), female animals often show better cognitive and behavioral recovery than male animals. In female animals, this advantage is conferred by higher levels of systemic progesterone at the time of injury. In rats, postmortem examination of brain tissue reveals that pseudo-pregnant female rats (high progesterone, low estrogen) have virtually no cerebral edema compared with male rats and low- progesterone-state normal cycling female rats after TBI. Progesterone injections can also eliminate cerebral edema in brain-injured male rats. Sex differences in the outcome of TBI highlight the importance of consid ering the timing of therapy and hormonal fluctuations in developing safe and effective treatments for CNS injury. NEUROSCIENTIST 4:435-442, 1998

Testosterone or oestradiol implants in the medial preoptic area induce mating in noncopulating male rats

Noncopulating (NC) male rats are those males that do not mount, intromit or ejaculate when repeatedly tested with receptive females. The lack of sexual behaviour in these males is not associated with alterations in testosterone or oestradiol (E2) plasma concentrations. Instead, it has been shown that androgen receptors are higher and oestrogen receptors are lower in the medial preoptic area (MPOA) of NC male rats than those observed in copulating (C) male rats. We have also observed reduced aromatase activity in the MPOA (but not in other brain regions) of NC male rats. The aim of the present study was to determine whether testosterone or E2 implants in the MPOA of NC male rats could induce sexual behaviour. Accordingly, in Experiment 1, we evaluated the long-term effects of testosterone or E2 implants in the MPOA, the ventromedial nucleus of the hypothalamus or the medial amygdala with respect to inducing sexual behaviour in castrated C male rats. Male rats were bilaterally implanted with a guide cannula, either empty or containing testosterone or E2. Starting 1 week later, all male rats were mated once weekly for 5 months. As described previously, the site where hormone implants most consistently induced sexual behaviour in castrated C male rats was the MPOA. Experiment 1 extended these findings showing that the males continued mating even 5 months after the implant. In the second experiment, NC males were implanted in the MPOA with a guide cannula empty or filled with testosterone or E2. One week after the testosterone or E2 implant, the percentage of males that mounted and intromitted started to increase and, 5 weeks after the implant, 50% of the subjects displayed mounts and intromissions. All NC males implanted with testosterone ejaculated consistently from week 11 after the implant until the end of testing (5 months), whereas all subjects implanted with E2 ejaculated from week 16 after the implant until the end of testing. These results support the hypothesis that, in the MPOA of NC male rats, there is a hormonal alteration associated with the lack of sexual behaviour.

Mechanisms involved in dual vasopressin/apelin neuron dysfunction during aging

Normal aging is associated with vasopressin neuron adaptation, but little is known about its effects on the release of apelin, an aquaretic peptide colocalized with vasopressin. We found that plasma vasopressin concentrations were higher and plasma apelin concentrations lower in aged rats than in younger adults. The response of AVP/apelin neurons to osmotic challenge was impaired in aged rats. The overactivity of vasopressin neurons was sustained partly by the increased expression of Transient receptor potential vanilloid2 (Trpv2), because central Trpv blocker injection reversed the age-induced increase in plasma vasopressin concentration without modifying plasma apelin concentration. The morphofunctional plasticity of the supraoptic nucleus neuron-astrocyte network normally observed during chronic dehydration in adults appeared to be impaired in aged rats as well. IL-6 overproduction by astrocytes and low-grade microglial neuroinflammation may contribute to the modification of neuronal functioning during aging. Indeed, central treatment with antibodies against IL-6 decreased plasma vasopressin levels and increased plasma apelin concentration toward the values observed in younger adults. Conversely, minocycline treatment (inhibiting microglial metabolism) did not affect plasma vasopressin concentration, but increased plasma apelin concentration toward control values for younger adults. This study is the first to demonstrate dual vasopressin/apelin adaptation mediated by inflammatory molecules and neuronal Trpv2, during aging.

Olfactory Hallucinations without Clinical Motor Activity: A Comparison of Unirhinal with Birhinal Phantosmia

Olfactory hallucinations without subsequent myoclonic activity have not been well characterized or understood. Herein we describe, in a retrospective study, two major forms of olfactory hallucinations labeled phantosmias: one, unirhinal, the other, birhinal. To describe these disorders we performed several procedures to elucidate similarities and differences between these processes. From 1272, patients evaluated for taste and smell dysfunction at The Taste and Smell Clinic, Washington, DC with clinical history, neurological and otolaryngological examinations, evaluations of taste and smell function, EEG and neuroradiological studies 40 exhibited cyclic unirhinal phantosmia (CUP) usually without hyposmia whereas 88 exhibited non-cyclic birhinal phantosmia with associated symptomology (BPAS) with hyposmia. Patients with CUP developed phantosmia spontaneously or after laughing, coughing or shouting initially with spontaneous inhibition and subsequently with Valsalva maneuvers, sleep or nasal water inhalation; they had frequent EEG changes usually ipsilateral sharp waves. Patients with BPAS developed phantosmia secondary to several clinical events usually after hyposmia onset with few EEG changes; their phantosmia could not be initiated or inhibited by any physiological maneuver. CUP is uncommonly encountered and represents a newly defined clinical syndrome. BPAS is commonly encountered, has been observed previously but has not been clearly defined. Mechanisms responsible for phantosmia in each group were related to decreased gamma-aminobutyric acid (GABA) activity in specific brain regions. Treatment which activated brain GABA inhibited phantosmia in both groups.

Hypothalamic control of energy balance: insights into the role of synaptic plasticity

The past 20 years witnessed an enormous leap in understanding of the central regulation of whole-body energy metabolism. Genetic tools have enabled identification of the region-specific expression of peripheral metabolic hormone receptors and have identified neuronal circuits that mediate the action of these hormones on behavior and peripheral tissue functions. One of the surprising findings of recent years is the observation that brain circuits involved in metabolism regulation remain plastic through adulthood. In this review, we discuss these findings and focus on the role of neurons and glial cells in the dynamic process of plasticity, which is fundamental to the regulation of physiological and pathological metabolic events.

Sexual differentiation of the brain and ADHD: what is a sex difference in prevalence telling us?

Sexual differentiation of the brain is a function of various processes that prepare the organism for successful reproduction in adulthood. Release of gonadal steroids during both the perinatal and the pubertal stages of development organizes many sex differences, producing changes in brain excitability and morphology that endure across the lifespan. To achieve these sexual dimorphisms, gonadal steroids capitalize on a number of distinct mechanisms across brain regions. Comparison of the developing male and female brain provides insight into the mechanisms through which synaptic connections are made, and circuits are organized that mediate sexually dimorphic behaviors. The prevalence of most psychiatric and neurological disorders differ in males versus females, including disorders of attention, activity and impulse control. While there is a strong male bias in incidence of attention deficit and hyperactivity disorders, the source of that bias remains controversial. By elucidating the biological underpinnings of male versus female brain development, we gain a greater understanding of how hormones and genes do and do not contribute to the differential vulnerability in one sex versus the other.

Neuronal plasticity and seasonal reproduction in sheep

Seasonal reproduction represents a naturally occurring example of functional plasticity in the adult brain as it reflects changes in neuroendocrine pathways controlling gonadotropin-releasing hormone (GnRH) secretion and, in particular, the responsiveness of GnRH neurons to estradiol negative feedback. Structural plasticity within this neural circuitry may, in part, be responsible for seasonal switches in the negative feedback control of GnRH secretion that underlie annual reproductive transitions. We review evidence for structural changes in the circuitry responsible for seasonal inhibition of GnRH secretion in sheep. These include changes in synaptic inputs onto GnRH neurons, as well as onto dopamine neurons in the A15 cell group, a nucleus that plays a key role in estradiol negative feedback. We also present preliminary data suggesting a role for neurotrophins and neurotrophin receptors as an early mechanistic step in the plasticity that accompanies seasonal reproductive transitions in sheep. Finally, we review recent evidence suggesting that kisspeptin cells of the arcuate nucleus constitute a critical intermediary in the control of seasonal reproduction. Although a majority of the data for a role of neuronal plasticity in seasonal reproduction has come from the sheep model, the players and principles are likely to have relevance for reproduction in a wide variety of vertebrates, including humans, and in both health and disease.

Organized for sex - steroid hormones and the developing hypothalamus

Steroid hormones of gonadal origin act on the neonatal brain, particularly the hypothalamus, to produce sex differences that underlie copulatory behavior. Neuroanatomical sex differences include regional volume, cell number, connectivity, morphology, physiology, neurotransmitter phenotype and molecular signaling, all of which are determined by the action of steroid hormones, particularly by estradiol in males, and are established by diverse downstream effects. Sex differences in distinct hypothalamic regions can be organized by the same steroid hormone, but the direction of a sex difference is often specific to one region or cell type, illustrating the wide range of effects that steroid hormones have on the developing brain. Substantial progress has been made in elucidating the downstream mechanisms through which gonadal hormones sexually differentiate the brain, but gaps remain in establishing the precise relationship between changes in neuronal morphology and behavior. A complete understanding of sexual differentiation will require integrating the diverse mechanisms across multiple brain regions into a functional network that regulates behavioral output.

Age-related decreases in SYN levels associated with increases in MAP-2, apoE, and GFAP levels in the rhesus macaque prefrontal cortex and hippocampus

Loss of synaptic integrity in the hippocampus and prefrontal cortex (PFC) may play an integral role in age-related cognitive decline. Previously, we showed age-related increases in the dendritic marker microtubule associated protein 2 (MAP-2) and the synaptic marker synaptophysin (SYN) in mice. Similarly, apolipoprotein E (apoE), involved in lipid transport and metabolism, and glial fibrillary acidic protein (GFAP), a glia specific marker, increase with age in rodents. In this study, we assessed whether these four proteins show similar age-related changes in a nonhuman primate, the rhesus macaque. Free-floating sections from the PFC and hippocampus from adult, middle-aged, and aged rhesus macaques were immunohistochemically labeled for MAP-2, SYN, apoE, and GFAP. Protein levels were measured as area occupied by fluorescence using confocal microscopy as well as by Western blot. In the PFC and hippocampus of adult and middle-aged animals, the levels of SYN, apoE, and GFAP immunoreactivity were comparable but there was a trend towards higher MAP-2 levels in middle-aged than adult animals. There was significantly less SYN and more MAP-2, apoE, and GFAP immunoreactivity in the PFC and hippocampus of aged animals compared to adult or middle-aged animals. Thus, the age-related changes in MAP-2, apoE, and GFAP levels were similar to those previously observed in rodents. On the other hand, the age-related changes in SYN levels were not, but were similar to those previously observed in the aging human brain. Taken together, these data emphasize the value of the rhesus macaque as a pragmatic translational model for human brain aging.

Function-related structural plasticity of the GnRH system: a role for neuronal-glial-endothelial interactions

As the final common pathway for the central control of gonadotropin secretion, GnRH neurons are subjected to numerous regulatory homeostatic and external factors to achieve levels of fertility appropriate to the organism. The GnRH system thus provides an excellent model in which to investigate the complex relationships between neurosecretion, morphological plasticity and the expression of a physiological function. Throughout the reproductive cycle beginning from postnatal sexual development and the onset of puberty to reproductive senescence, and even within the ovarian cycle itself, all levels of the GnRH system undergo morphological plasticity. This structural plasticity within the GnRH system appears crucial to the timely control of reproductive competence within the individual, and as such must have coordinated actions of multiple signals secreted from glial cells, endothelial cells, and GnRH neurons. Thus, the GnRH system must be viewed as a complete neuro-glial-vascular unit that works in concert to maintain the reproductive axis.

GABA and neuroactive steroid interactions in glia: new roles for old players?

In recent years it has becoming clear that glial cells of the central and peripheral nervous system play a crucial role from the earliest stages of development throughout adult life. Glial cells are important for neuronal plasticity, axonal conduction and synaptic transmission. In this respect, glial cells are able to produce, uptake and metabolize many factors that are essential for neuronal physiology, including classic neurotransmitters and neuroactive steroids. In particular, neuroactive steroids, which are mainly synthesized by glial cells, are able to modulate some neurotransmitter receptors affecting both glia and neurons. Among the signaling systems that are specialized for neuron-glial communication, we can include neurotransmitter GABA.The main focus of this review is to illustrate the cross-talk between neurons and glial cells in terms of GABA neurotransmission and actions of neuroactive steroids. To this purpose, we will review the presence of the different GABA receptors in the glial cells of the central and peripheral nervous system. Then, we will discuss their modulation by some neuroactive steroids.

Phosphorylation of N-methyl-D-aspartic acid receptor-associated neuronal nitric oxide synthase depends on estrogens and modulates hypothalamic nitric oxide production during the ovarian cycle

Within the preoptic region, nitric oxide (NO) production varies during the ovarian cycle and has the ability to impact hypothalamic reproductive function. One mechanism for the regulation of NO release mediated by estrogens during the estrous cycle includes physical association of the calcium-activated neuronal NO synthase (nNOS) enzyme with the glutamate N-methyl-d-aspartate (NMDA) receptor channels via the postsynaptic density 95 scaffolding protein. Here we demonstrate that endogenous variations in estrogens levels during the estrous cycle also coincide with corresponding changes in the state of nNOS Ser1412 phosphorylation, the level of association of this isoform with the NMDA receptor/postsynaptic density 95 complex at the plasma membrane, and the activity of NO synthase (NOS). Neuronal NOS Ser1412 phosphorylation is maximal on the afternoon of proestrus when both the levels of estrogens and the physical association of nNOS with NMDA receptors are highest. Estradiol mimicked these effects in ovariectomized (OVX) rats. In addition, the catalytic activity of NOS in membrane protein extracts from the preoptic region, i.e. independent of any functional protein-protein interactions or cell-cell signaling, was significantly increased in estradiol-treated OVX rats compared with OVX rats. Finally, lambda phosphatase-mediated nNOS dephosphorylation dramatically impaired NOS activity in preoptic region protein extracts, thus demonstrating the important role of phosphorylation in the regulation of NO production in the preoptic region. Taken together, these results yield new insights into the regulation of neuron-derived NO production by gonadal steroids within the preoptic region and raise the possibility that changes in nNOS phosphorylation during fluctuating physiological conditions may be involved in the hypothalamic control of key neuroendocrine functions, such as reproduction.

Sex differences and laterality in astrocyte number and complexity in the adult rat medial amygdala

The posterodorsal portion of the medial amygdala (MePD) is sexually dimorphic in several rodent species. In several other brain nuclei, astrocytes change morphology in response to steroid hormones. We visualized MePD astrocytes using glial-fibrillary acidic protein (GFAP) immunocytochemistry. We compared the number and process complexity of MePD astrocytes in adult wildtype male and female rats and testicular feminized mutant (TFM) male rats that lack functional androgen receptors (ARs) to determine whether MePD astrocytes are sexually differentiated and whether ARs have a role. Unbiased stereological methods revealed laterality and sex differences in MePD astrocyte number and complexity. The right MePD contained more astrocytes than the left in all three genotypes, and the number of astrocytes was also sexually differentiated in the right MePD, with males having more astrocytes than females. In contrast, the left MePD contained more complex astrocytes than did the right MePD in all three genotypes, and males had more complex astrocytes than females in this hemisphere. TFM males were comparable to wildtype females, having fewer astrocytes on the right and simpler astrocytes on the left than do wildtype males. Taken together, these results demonstrate that astrocytes are sexually dimorphic in the adult MePD and that the nature of the sex difference is hemisphere-dependent: a sex difference in astrocyte number in the right MePD and a sex difference in astrocyte complexity in the left MePD. Moreover, functional ARs appear to be critical in establishing these sex differences in MePD astrocyte morphology.

Steroid-induced sexual differentiation of the developing brain: multiple pathways, one goal

Hormone exposure, including testosterone and its metabolite estradiol, induces a myriad of effects during a critical period of brain development that are necessary for brain sexual differentiation. Nuclear volume, neuronal morphology, and astrocyte complexity are examples of the wide range of effects by which testosterone and estradiol can induce permanent changes in the function of neurons for the purpose of reproduction in adulthood. This review will examine the multitude of mechanisms by which steroid hormones induce these permanent changes in brain structure and function. Elucidating how steroids alter brain development sheds light on how individual variation in neuronal phenotype is established during a critical period.

Cellular mechanisms of estradiol-mediated masculinization of the brain

The sexual differentiation of reproductive physiology and behavior in the rodent brain is largely determined by estradiol aromatized from testicular androgens. The cellular mechanisms by which estradiol masculinizes the brain are beginning to emerge and revealing novel features of brain development that are highly region-specific. In the preoptic area, the major site controlling male sexual behavior, estradiol increases the level of the COX-2 enzyme and its product, prostaglandin E2 which promotes dendritic spine synaptogenesis. In the ventromedial nucleus of the hypothalamus, the major site controlling female reproductive behavior, estradiol promotes glutamate release from synaptic terminals, activating NMDA receptors and the MAP kinase pathway. In the arcuate nucleus, a major regulator of anterior pituitary function, estradiol increases GABA synthesis, altering the morphology of neighboring astrocytes and reducing formation of dendritic spines synapses. Glutamate, GABA and the importance of neuronal-astrocytic cross-talk are emerging as common aspects of masculinization. Advances are also being made in the mechanistic basis of female brain development, although the challenges are far greater.

Estradiol and the developing brain

Estradiol is the most potent and ubiquitous member of a class of steroid hormones called estrogens. Fetuses and newborns are exposed to estradiol derived from their mother, their own gonads, and synthesized locally in their brains. Receptors for estradiol are nuclear transcription factors that regulate gene expression but also have actions at the membrane, including activation of signal transduction pathways. The developing brain expresses high levels of receptors for estradiol. The actions of estradiol on developing brain are generally permanent and range from establishment of sex differences to pervasive trophic and neuroprotective effects. Cellular end points mediated by estradiol include the following: 1) apoptosis, with estradiol preventing it in some regions but promoting it in others; 2) synaptogenesis, again estradiol promotes in some regions and inhibits in others; and 3) morphometry of neurons and astrocytes. Estradiol also impacts cellular physiology by modulating calcium handling, immediate-early-gene expression, and kinase activity. The specific mechanisms of estradiol action permanently impacting the brain are regionally specific and often involve neuronal/glial cross-talk. The introduction of endocrine disrupting compounds into the environment that mimic or alter the actions of estradiol has generated considerable concern, and the developing brain is a particularly sensitive target. Prostaglandins, glutamate, GABA, granulin, and focal adhesion kinase are among the signaling molecules co-opted by estradiol to differentiate male from female brains, but much remains to be learned. Only by understanding completely the mechanisms and impact of estradiol action on the developing brain can we also understand when these processes go awry.

Effect of gender on mitochondrial toxicity of Alzheimer's Abeta peptide

The aim of this article is to review the role of mitochondria in the pathogenesis of Alzheimer's disease. Additionally, the effect of gender on the incidence of Alzheimer's disease and the pathophysiological mechanisms involved will be discussed. Mitochondria, in the presence of Alzheimer's amyloid-beta peptide, increase the formation of reactive oxygen species which act both as damaging agents and also as signaling molecules. These radicals, in fact, unleash a mechanism involving the liberation of cytochrome c that leads to neuronal apoptosis. Notably, young females appear protected against the mitochondrial toxicity of amyloid-beta, likely due to the upregulation of antioxidant enzymes which occur in females. Estrogens are responsible for this effect. Overall, the findings support the notion that amyloid-beta causes intracellular toxicity via the increased production of oxidant species. Reactive oxygen species generated by mitochondria act as a signal to start the mitochondrial apoptotic pathway. There is a possibility of prevention, and indirect evidence shows that estrogenic compounds (either endogenous estradiol or phytoestrogens such as genistein) may increase the expression of antioxidant enzymes, leading to a lowering of oxidative stress and thus protection against intracellular toxicity of amyloid-beta peptide. These ideas open up the possibility of using phytoestrogens to prevent the onset of Alzheimer's disease. More studies are required to determine whether estrogens and/or phytoestrogens fulfill these expectations.

Cajal's contributions to glia research

In 1906, Santiago Ramón y Cajal was awarded the Nobel Prize in Physiology or Medicine in recognition of his work on the structure of neurons and their connections. What is less well known is that he also had a keen interest in glia and developed specific staining methods for their study. In addition to describing their morphology, he speculated on a role for glia in sleep and wakefulness and even in executive brain functions such as attention. In this article, we focus on Cajal's histological research into glial cells; this research includes original drawings of astrocytes, oligodendrocytes, microglia and radial glia, as well as his scientific writings. We aim to show that, concerning glia as well as neurons, Cajal was far ahead of his time.

The role of insulin-like growth factor-I and growth factor-associated signal transduction pathways in estradiol and progesterone facilitation of female reproductive behaviors

We are examining the role of insulin-like growth factor-I (IGF-I) and downstream signal transduction pathways associated with growth factors (e.g., mitogen-activated protein kinase, MAPK) in estradiol and progesterone facilitation of female reproductive behavior in rats. Brain IGF-I receptor activity is required for the long-term, priming actions of estradiol on the female reproductive axis. Infusions of an IGF-I receptor antagonist during estradiol priming blocks induction of hypothalamic alpha(1B)-adrenergic receptors and luteinizing hormone surges, and attenuates lordosis behavior. Infusion of MAPK and phosphatidylinositol-3-kinase inhibitors inhibitors during estradiol priming completely blocks hormone-facilitated lordosis. Because progestin receptors (PRs) can be phosphorylated and activated by MAPKs, growth factor signaling pathways may also participate in progesterone facilitation of reproductive behaviors. Infusion of a MAPK inhibitor in estradiol-primed rats blocks progestin facilitation and sequential inhibition of lordosis and proceptive behaviors. Interference with MAPK signaling also inhibits behavioral responses to cGMP and a delta-opioid agonist, both of which can activate MAPK in some cells. Thus MAPK is involved in the facilitation of lordosis and proceptive behaviors, perhaps by phosphorylation of hypothalamic PRs.

Estrogen, cognition and female ageing

Starting from fetal life, estrogens are crucial in determining central gender dimorphism, and an estrogen-induced synaptic plasticity is well evident during puberty and seasonal changes as well as during the ovarian cycle. Estrogens act on the central nervous system (CNS) both through genomic mechanisms, modulating synthesis, release and metabolism of neurotransmitters, neuropeptides and neurosteroids, and through non-genomic mechanisms, influencing electrical excitability, synaptic function and morphological features. Therefore, estrogen's neuroactive effects are multifaceted and encompass a system that ranges from the chemical to the biochemical to the genomic mechanisms, protecting against a wide range of neurotoxic insults. Clinical evidences show that, during the climacteric period, estrogen withdrawal in the limbic system gives rise to modifications in mood, behaviour and cognition and that estrogen administration is able to improve mood and cognitive efficiency in post-menopause. Many biological mechanisms support the hypothesis that estrogens might protect against Alzheimer's disease (AD) by influencing neurotransmission, increasing cerebral blood flow, modulating growth proteins associated with axonal elongation and blunting the neurotoxic effects of beta-amyloid. On the contrary, clinical studies of estrogen replacement therapy (ERT) and cognitive function have reported controversial results, indicating a lack of efficacy of estrogens on cognition in post-menopausal women aged >or=65 years. These findings suggest the presence of a critical period for HRT-related neuroprotection and underlie the potential importance of early initiation of therapy for cognitive benefit. In this review, we shall first describe the multiple effects of steroids in the nervous system, which may be significant in the ageing process. A critical update of HRT use in women and a discussion of possible prospectives for steroid use are subsequently proposed.

Steroids and glial cell function

Hormonal and locally produced steroids act in the nervous system as neuroendocrine regulators, as trophic factors and as neuromodulators and have a major impact on neural development and function. Glial cells play a prominent role in the local production of steroids and in the mediation of steroid effects on neurons and other glial cells. In this review, we examine the role of glia in the synthesis and metabolism of steroids and the functional implications of glial steroidogenesis. We analyze the mechanisms of steroid signaling on glia, including the role of nuclear receptors and the mechanisms of membrane and cytoplasmic signaling mediated by changes in intracellular calcium levels and activation of signaling kinases. Effects of steroids on functional parameters of glia, such as proliferation, myelin formation, metabolism, cytoskeletal reorganization, and gliosis are also reviewed, as well as the implications of steroid actions on glia for the regulation of synaptic function and connectivity, the regulation of neuroendocrine events, and the response of neural tissue to injury.

Sex differences in neuronal morphology in the killifish hypothalamus

This study examined the neuroarchitecture of the male and female killifish (Fundulus heteroclitus) hypothalamus to evaluate whether sexual dimorphism of this brain region exists in fishes as it does in mammals and other vertebrates. The rostral medulla, a brain region distinct from the hypothalamic-pituitary-gonadal axis, was also examined to determine if any observed differences were region-specific. With the use of Golgi-Cox impregnation, five dendritic characteristics were measured from neurons of both the hypothalamus and medulla including: spine density, number of branch points, dendrite length, surface area and volume. Dendritic spines are associated with excitatory synapses, and changes in density are associated with a variety of normal and pathological changes. Consistent with mammalian studies, we found that adult female killifish have 25% greater dendritic spine densities in the hypothalamus than male killifish (densities of 0.34+/-0.06 microm-1 and 0.25+/-0.08 microm-1, respectively). By contrast, no statistically significant difference between males and females was detected in spine densities in the rostral medulla. This finding supports the conclusion that hypothalamic sexual dimorphism is conserved in killifish.

Synaptic remodeling induced by gonadal hormones: neuronal plasticity as a mediator of neuroendocrine and behavioral responses to steroids

During recent decades, it has become a generally accepted view that structural neuroplasticity is remarkably involved in the functional adaptation of the CNS. Thus, cellular morphology in the brain is in continuous transition throughout the life span, as a response to environmental stimuli. The effects of the environment on neuroplasticity are mediated by, to some extent, the changing levels of circulating gonadal steroid hormones. Today, it is clear that the function of gonadal steroids in the brain extends beyond simply regulating reproductive and/or neuroendocrine events. In addition, or even more importantly, gonadal steroids participate in the shaping of the developing brain, while their actions during adult life are implicated in higher brain functions such as cognition, mood and memory. A large body of evidence indicates that gonadal steroid-induced functional changes are accompanied by alterations in neuron and synapse numbers, as well as in dendritic and synaptic morphology. These structural modifications are believed to serve as a morphological basis for changes in behavior and cellular activity. Due to their growing functional and clinical significance, the specificity, timeframe, as well as the molecular and cellular mechanisms of hormone-induced neuroplasticity have become the focus of many studies. In this review, we briefly summarize current knowledge and the most significant recent discoveries from our laboratories on estrogen- and dehydroepiandrosterone-induced synaptic remodeling in the hypothalamus and hippocampus, two important brain areas heavily involved in autonomic and cognitive operations, respectively.

Administration of 17alpha-ethinylestradiol during pregnancy elicits modifications of maternal behavior and emotional alteration of the offspring in the rat

Intraperitoneal administration of 17alpha-ethinylestradiol (15 microg.kg(-1)) in pregnant rats, every day from day 9 to day 14 of pregnancy, elicited a high percentage of abortions. Quantification of maternal behavior showed that treated dams took better care of their pups than control dams, injected with the vehicle only, did. Postnatal reflexes, which reflect maturational rate, were established more promptly in the offspring of treated dams than in the offspring of control dams. However, when adult, the rats born from treated dams developed anxiety- and depressive-like behaviors. All these results are explained by the effects of the exogenous estrogen on the developing brain of the fetuses.

Estradiol inhibits GSK3 and regulates interaction of estrogen receptors, GSK3, and beta-catenin in the hippocampus

Estrogens regulate a wide set of neuronal functions such as gene expression, survival and differentiation in a manner not very different from that exerted by neurotrophins or by growth factors. The best-studied hormonal action is the transcriptional activation mediated by estrogen receptors. However, the direct effects of estrogen on growth factor signaling have not been well clarified. The present data show that estradiol, in vivo, induces a transient activation of GSK3 in the adult female rat hippocampus, followed by a more sustained inhibition, as inferred from phosphorylation levels of Tau. Similar data was obtained from cultured hippocampal neurons when treated with the hormone. The transient activation was confirmed by direct measure of GSK3 kinase activity. In addition, our results show a novel complex of estrogen receptor alpha, GSK3, and beta-catenin. The presence of the hormone removes beta-catenin from this complex. There is a second complex, also affected by estradiol, in which Tau is associated with GSK3, beta-catenin, and elements of the PI3 kinase complex. Considering the role of GSK3 in neurodegeneration, our data suggest that part of the neuroprotective effects of estrogen may be due to the control of GSK3.

Testosterone regulates BCL-2 immunoreactivity in a sexually dimorphic motor pool of adult rats

Bcl-2 and Bax immunoreactivity were examined in the spinal nucleus of the bulbocavernosus (SNB), an androgen-sensitive motor pool of adult rats. Castration reduced Bcl-2 immunoreactivity and testosterone treatment of castrates prevented this decline. Hormone manipulations did not affect Bcl-2 or Bax staining in the retrodorsolateral nucleus (RDLN), a relatively androgen-insensitive nucleus at the same spinal level. Changes in Bcl-2 expression may underlie the hormonal control of cell death and/or neural plasticity in SNB motoneurons.

Steroid modulation of astrocytes in the neonatal brain: implications for adult reproductive function

There is a growing appreciation for the importance of astrocytes, a type of nonneuronal glial cell, to overall brain functioning. The ability of astrocytes to respond to gonadal steroid hormones with changes in morphology has been well documented in the adult brain. It is also apparent that astrocytes of the developing brain are permanently differentiated by the neonatal hormonal milieu, in particular by estradiol, resulting in sexually dimorphic cell morphology, synaptic patterning, and density in males and females. The mechanisms of hormonally mediated astrocyte differentiation are likely to be region specific. In the arcuate nucleus of the hypothalamus, neuron-to-astrocyte signaling appears to play a critical role in estradiol-induced astrocyte differentiation during the first few days of life. Gamma aminobutyric acid (GABA) is an amino acid neurotransmitter that is synthesized and released exclusively by neurons. The levels of GABA are increased in the arcuate nucleus of neonatal males versus females. Preventing the increase in males or mimicking GABA action in females modulates astrocytes accordingly. Speculation about and evidence in support of the functional significance of this dimorphism to adult reproductive functioning is the topic of this review.

Perspective: micoarrays and differential display PCR-tools for studying transcript levels of genes in neuroendocrine systems

A central goal of neuroendocrinology is the understanding of how hormones modulate a variety of neurobiological functions including releasing factors for anterior pituitary secretions and behavior. We know that mechanisms of hormone actions clearly include the activation and repression of genes either directly through nuclear hormone receptors or indirectly, through a series of transduced signals originating from membrane receptors. Until recently, identification of the differentially expressed genes has been a "gene-at-a-time" proposition. With the advent of the completion of sequencing of several genomes including those of the human and mouse, new methods for the simultaneous assessment of many genes' expression are proving especially timely. Two such methods, differential display PCR and gene microarrays, are based on the well-established principles of DNA amplification and nucleic acid hybridization, respectively. With properly designed and well-executed experiments, these methods are powerful tools in the assessment of differentially expressed genes yielding results both expected and unanticipated.

Effects of gonadal hormones and persistent pain on non-spatial working memory in male and female rats

There are indications of a modulatory role carried out by gonadal hormones and pain in cognitive functions. We have examined this issue in male and female rats by assessing the impact of gonadectomy and persistent pain on the object recognition test. Intact and gonadectomized male and female rats were exposed to an open field (15 min) in which three objects were placed (Trial 1); the same test was repeated 2 h later (Trial 2), after the replacement of a "familiar" object with a novel one. Three days later (Day 2), the same procedure was repeated (Trial 3 and 4 with 2 h in between) but half of the animals were exposed to formalin-injection immediately before Trial 3. The latency, frequency and duration of approaching the three objects were recorded in each trial and compared by sex, gonadectomy and formalin treatment. The results showed that gonadectomized males and females had lower levels of approach to all objects and less locomotor/exploratory activity than intact animals in all experimental trials; their behaviour was not affected by repetition of the test or by pain. On Day 1, intact males showed a higher level of approach to the novel object than females. In intact males, the 2 h delay between the first and second trial failed to induce any significant modification of exploration of the novel object with respect to the familiar one, while in intact females the novel object was approached much less than the familiar one. Similarly on Day 2, the novel object was approached for a longer time by intact males than by all the other groups. In conclusion, our data show that physiological levels of circulating gonadal hormones significantly affected the performance of male but not female rats when exposed to the object recognition test.

Prenatal exposure to ethinylestradiol elicits behavioral abnormalities in the rat

Pregnant rats were i.p. injected with a solution of 17alpha-ethinylestradiol (15 microg kg(-1)) every day between day 9 and day 14 of pregnancy and the behavior of the offspring was compared to that of rats born from dams injected with the vehicle only during the same gestational period. The percentage of neonatal death was dramatically high in the prenatally treated group. Growth of the surviving animals was even better than that of controls, but when adult, they exhibited a number of behavioral abnormalities: increased spontaneous motor activity, decreased exploratory behavior, impaired cognitive processing, qualitatively different exploratory drive, and/or persevering behavior, increased anxiety-like behavior and social neophobia. These behavioral alterations, which resemble a number of psychiatric syndromes, suggest that ethinylestradiol altered the ontogenesis of different parts of the central nervous system involved in cognitive and emotional processes. However, it cannot be excluded that the changes in behavior of ethinylestradiol exposed offspring were due to the abnormal maternal behavior of the estradiol treated dams.

Gender, neuroendocrine-immune interactions and neuron-glial plasticity. Role of luteinizing hormone-releasing hormone (LHRH)

Signals generated by the hypothalamic-pitutary-gonadal (HPG) axis powerfully modulate immune system function. This article summarizes some aspects of the impact of gender in neuroendocrine immunomodulation. Emphasis is given to the astroglial cell compartment, defined as a key actor in neuroendocrine immune communications. In the brain, the principal hormones of the HPG axis directly interact with astroglial cells. Thus, luteinizing hormone releasing hormone, LHRH, influences hypothalamic astrocyte development and growth, and hypothalamic astrocytes direct LHRH neuron differentiation. Hormonally induced changes in neuron-glial plasticity may dictate major changes in CNS output, and thus actively participate in sex dimorphic immune responses. The impact of gender in neuroimmunomodulation is further underlined by the sex dimorphism in the expression of genes encoding for neuroendocrine hormones and their receptors within the thymus, and by the potent modulation exerted by circulating sex steroids during development and immunization. The central role of glucocorticoids in the interactive communication between neuroendocrine and immune systems, and the impact of gender on hypothalamic-pituitary-adrenocortical (HPA) axis modulation is underscored in transgenic mice expressing a glucocorticoid receptor antisense RNA.

Duration and amplitude of the luteal phase progesterone increment times the estradiol-induced luteinizing hormone surge in ewes

Progesterone (P) powerfully inhibits the neuroendocrine reproductive axis, but the mechanisms and site or sites of action of this steroid remain poorly understood. Progesterone exposure during the luteal phase also alters the responsiveness of the hypothalamus to increased concentrations of estrogen (E) during the follicular phase. Using an ovariectomized ovine follicular phase model, we investigated whether the amplitude and duration of the luteal phase increase in circulating P affects the E-induced surge in LH. Treatment of ewes for 10 days with two, one, or half an intravaginal P-releasing implant or with an empty implant demonstrated that P concentrations significantly (P: < 0.0001) delayed the time to surge onset upon exposure to an equal concentration of E. This delay was not due to a time-related difference in responsiveness to E after P clearance because the time of surge onset was not different when E treatment began 6, 12, or 24 h after the withdrawal of two P implants that had been present for 10 days. The final study demonstrated that the duration of P before treatment (5, 10, or 30 days) significantly (P: < 0.0001) delayed the responsiveness of the estradiol-dependent surge-generating system. There was no effect on surge amplitude or duration in any experiment. Thus, the amplitude and duration of exposure to luteal phase P significantly affect the neural elements targeted by E to induce the preovulatory LH surge.

Memory functioning at menopause: impact of age in ovariectomized women

Estrogens are known to act selectively on some components of memory, exerting beneficial effects on cognitive performances. However, there are few data on the long-term effect of the lack of estrogen in postmenopausal women. Therefore, we investigated attentive and verbal memory performances in physiological and surgical menopause, drawing attention to the impact of age at menopause, and we compared a well-known aging and estrogen-dependent index, the entity of bone mass loss to memory functioning. No significant differences were found in the mean scores of attentive and psychomotor performances between physiological and surgical menopause, whereas a lower number of recalled words (recency effect = PS2) was found in surgical menopause (p < 0.001) in comparison to physiological menopause. In addition, both the age at the time of ovariectomy (r = 0.47; p = 0. 014) and the years since surgery (r = -0.64; p = 0.000) correlated to short-term verbal memory performance (PS2) with better scores when surgery occurred later in women's lives. Surgical menopause is able to affect short-term verbal memory more than physiological menopause and seems to represent a critical negative event within the female brain, in particular when it occurs prematurely.

Differential effect of oestradiol and astroglia-conditioned media on the growth of hypothalamic neurons from male and female rat brains

To determine whether soluble products from different CNS regions differ in their ability to support oestrogen-stimulated neurite growth, hypothalamic neurons from sexually segregated embryos were cultured with astroglia-conditioned medium (CM) derived from cortex, striatum and mesencephalon, with or without 17-beta-oestradiol 100 nM added to the medium. After 48 h in vitro, neurite outgrowth was quantified by morphometric analysis. Astroglia-CM from mesencephalon (a target for the axons of hypothalamic neurons) induced the greatest axogenic response in males and in this case only a neuritogenic effect could be demonstrated for oestradiol. On the other hand, astroglia-CM from regions that do not receive projections from ventromedial hypothalamus inhibited axon growth. A sexual difference in the response of hypothalamic neurons to astroglia-CM and oestradiol was found; growth of neurons from female foetuses was increased by astroglia-CM from mesencephalon, but no neuritogenic effect could be demonstrated for oestradiol in these cultures. Blot immunobinding demonstrated the presence of receptors for neurotrophic factors in cultures of hypothalamic neurons; Western blot analysis of these cultures demonstrated that oestradiol increased the concentration of trkB and IGF-I Rbeta, whereas trkA was not detected and the concentration of trkC was not modified. These results support the hypothesis that target regions produce some factor(s) that stimulate the growth of axons from projecting neurons and further indicate that in the case of males this effect is modulated by oestradiol, perhaps mediated through the upregulation of trkB and IGF-I receptors.

Gonadal steroids promote glial differentiation and alter neuronal morphology in the developing hypothalamus in a regionally specific manner

One of the more striking sexual dimorphisms in the adult brain is the synaptic patterning in some hypothalamic nuclei. In the arcuate nucleus (ARC) males have twice the number of axosomatic and one-half the number of axodendritic spine synapses as females. The opposite pattern is observed in the immediately adjacent ventromedial nucleus (VMN). In both cases, early exposure to testosterone dictates adult dimorphism, but the exact timing, mechanism, and site of steroid action remain unknown. Astrocytes also exhibit sexual dimorphisms, and their role in mediating neuronal morphology is becoming increasingly evident. Using Golgi-Cox impregnation to examine neuronal morphology and glial fibrillary acidic protein immunoreactivity (GFAP-IR) to characterize astrocytic morphology, we compared structural differences in dendrites and astrocytes from the ARC and VMN in postnatal day 2 rat pups from four hormonally different groups. Consistent with previous observations, testosterone exposure induced a rapid and dramatic stellation response in ARC astrocytes. Coincident with this change in astrocytic morphology was a 37% reduction in the density of dendritic spines on ARC neurons. In contrast, astrocytes in the VMN were poorly differentiated and did not respond to testosterone exposure, nor were there any changes in neuronal dendrite spine density. However, VMN neurons exposed to testosterone had almost double the number of branches compared with that in controls. These data suggest that the degree of maturation and the differentiation of hypothalamic astrocytes in vivo are correlated with the ability of neurons to sprout branches or spines in response to steroid hormones and may underlie regionally specific differences in synaptic patterning.