Intracellular signalling systems controlling the 5 alpha-reductase in glial cell cultures

Neuropeptidergic control of neurosteroids biosynthesis

Neurosteroids are steroids synthesized within the central nervous system either from cholesterol or by metabolic reactions of circulating steroid hormone precursors. It has been suggested that neurosteroids exert pleiotropic activities within the central nervous system, such as organization and activation of the central nervous system and behavioral regulation. It is also increasingly becoming clear that neuropeptides exert pleiotropic activities within the central nervous system, such as modulation of neuronal functions and regulation of behavior, besides traditional neuroendocrinological functions. It was hypothesized that some of the physiological functions of neuropeptides acting within the central nervous system may be through the regulation of neurosteroids biosynthesis. Various neuropeptides reviewed in this study possibly regulate neurosteroids biosynthesis by controlling the activities of enzymes that catalyze the production of neurosteroids. It is now required to thoroughly investigate the neuropeptidergic control mechanisms of neurosteroids biosynthesis to characterize the physiological significance of this new neuroendocrinological phenomenon.

Biosynthesis and signalling functions of central and peripheral nervous system neurosteroids in health and disease

Neurosteroids are steroid hormones synthesised de novo in the brain and peripheral nervous tissues. In contrast to adrenal steroid hormones that act on intracellular nuclear receptors, neurosteroids directly modulate plasma membrane ion channels and regulate intracellular signalling. This review provides an overview of the work that led to the discovery of neurosteroids, our current understanding of their intracellular biosynthetic machinery, and their roles in regulating the development and function of nervous tissue. Neurosteroids mediate signalling in the brain via multiple mechanisms. Here, we describe in detail their effects on GABA (inhibitory) and NMDA (excitatory) receptors, two signalling pathways of opposing function. Furthermore, emerging evidence points to altered neurosteroid function and signalling in neurological disease. This review focuses on neurodegenerative diseases associated with altered neurosteroid metabolism, mainly Niemann-Pick type C, multiple sclerosis and Alzheimer disease. Finally, we summarise the use of natural and synthetic neurosteroids as current and emerging therapeutics alongside their potential use as disease biomarkers.

Overview of the Molecular Steps in Steroidogenesis of the GABAergic Neurosteroids Allopregnanolone and Pregnanolone

Allopregnanolone and pregnanolone-neurosteroids synthesized from progesterone in the brain, adrenal gland, ovary and testis-have been implicated in a range of neuropsychiatric conditions including seizure disorders, post-traumatic stress disorder, major depression, post-partum depression, pre-menstrual dysphoric disorder, chronic pain, Parkinson's disease, Alzheimer's disease, neurotrauma, and stroke. Allopregnanolone and pregnanolone equipotently facilitate the effects of gamma-amino-butyric acid (GABA) at GABAA receptors, and when sulfated, antagonize N-methyl-D-aspartate receptors. They play myriad roles in neurophysiological homeostasis and adaptation to stress while exerting anxiolytic, antidepressant, anti-nociceptive, anticonvulsant, anti-inflammatory, sleep promoting, memory stabilizing, neuroprotective, pro-myelinating, and neurogenic effects. Given that these neurosteroids are synthesized de novo on demand, this review details the molecular steps involved in the biochemical conversion of cholesterol to allopregnanolone and pregnanolone within steroidogenic cells. Although much is known about the early steps in neurosteroidogenesis, less is known about transcriptional, translational, and post-translational processes in allopregnanolone- and pregnanolone-specific synthesis. Further research to elucidate these mechanisms as well as to optimize the timing and dose of interventions aimed at altering the synthesis or levels of these neurosteroids is much needed. This should include the development of novel therapeutics for the many neuropsychiatric conditions to which dysregulation of these neurosteroids contributes.

Androgen cell signaling pathways involved in neuroprotective actions

As a normal consequence of aging in men, testosterone levels significantly decline in both serum and brain. Age-related testosterone depletion results in increased risk of dysfunction and disease in androgen-responsive tissues, including brain. Recent evidence indicates that one deleterious effect of age-related testosterone loss in men is increased risk for Alzheimer's disease (AD). We discuss recent findings from our laboratory and others that identify androgen actions implicated in protecting the brain against neurodegenerative diseases and begin to define androgen cell signaling pathways that underlie these protective effects. Specifically, we focus on the roles of androgens as (1) endogenous negative regulators of beta-amyloid accumulation, a key event in AD pathogenesis, and (2) neuroprotective factors that utilize rapid non-genomic signaling to inhibit neuronal apoptosis. Continued elucidation of cell signaling pathways that contribute to protective actions of androgens should facilitate the development of targeted therapeutic strategies to combat AD and other age-related neurodegenerative diseases.

Flutamide and cyproterone acetate exert agonist effects: induction of androgen receptor-dependent neuroprotection

Androgens can exert profound effects on the organization, development, and function of the nervous system through activation of androgen receptors (ARs). Nonsteroidal and steroidal antiandrogens antagonize AR-mediated, classic genomic actions of androgens. However, emerging studies in nonneuronal cells indicate that antiandrogens can act as partial agonists for the AR. Here we investigated the effects of the antiandrogens flutamide and cyproterone acetate on neuroprotection induced by dihydrotestosterone (DHT). We observed that, although flutamide and cyproterone acetate blocked androgen-induced gene expression, they failed to inhibit DHT protection against apoptotic insults in cultured hippocampal neurons. Interestingly, flutamide and cyproterone acetate alone, like DHT, significantly reduced apoptosis. Furthermore, the protective actions of flutamide and cyproterone acetate were observed specifically in AR-expressing cell lines, suggesting a role for AR in the agonist effects of antiandrogens. Our results indicate that, in contrast to the classic antiandrogen properties of flutamide and cyproterone acetate, these AR modulators display agonist activities at the level of neuroprotection. These findings provide new insight into the agonist vs. antagonist properties of antiandrogens, information that will be crucial to understanding the neural implications of clinically used AR-modulating drugs.

Neurosteroids, neuroactive steroids, and symptoms of affective disorders

Neurosteroids (NS) are steroids synthesized by the brain. Neuroactive steroids (NAS) refers to steroids that, independent of their origin, are capable of modifying neural activities. NAS bind and modulate different types of membrane receptors. The gamma amino butyric acid (GABA) and sigma receptor complexes have been the most extensively studied. Oxidized ring A reduced pregnanes, tetrahydroprogesterone (THP), and tetrahydrodeoxycorticosterone (THDOC) bind to the progesterone intracellular receptor (PR), and in this way can also regulate gene expression. Animal experimentation showed that salient symptoms of depression, viz., anxiety, sleep disturbances, and memory and sexual dysfunctions, are modulated by NAS. In turn, psychotropic drugs modulate NS and NAS levels. NS levels as well as NAS plasma concentrations change in patients with depression syndromes, the levels return to normal baseline with recovery, but normalization is not necessary for successful therapy. Results from current studies on the evolution of nervous systems, including evolutionary developmental biology as well as anatomical and physiological findings, almost preclude a categorical classification of the psychiatric ailments the human brain succumbs to. The persistence in maintaining such essentialist classifications may help to explain why up to now the search for biological markers in psychiatry has been an unrewarding effort. It is proposed that it would be more fruitful to focus on relationships between NAS and symptoms of psychiatric disorders, rather than with typologically defined disorders.

Possible involvement of 5α-reduced neurosteroids in adrenergic and serotonergic stimulation of GFAP gene expression in rat C6 glioma cells

Influence of adrenergic and serotonergic stimulation on glial fibrillary acidic protein (GFAP) gene expression in rat C6 glioma cells was first examined as an in vitro model experiment for investigating the neuronal regulation of glial cell differentiation. Stimulation of these cells with isoproterenol and serotonin elevated GFAP mRNA levels followed by an increase in its protein contents, thus suggesting that both adrenergic and serotonergic stimulation might induce the differentiation of the glioma cells. In addition, progesterone and its 5alpha-reduced metabolite dihydroprogesterone also elevated GFAP mRNA levels in rat C6 glioma cells, consistent with their stimulatory actions on GFAP gene expression observed in rat astrocytes. Further studies showed that the elevation of GFAP mRNA levels induced by isoproterenol and serotonin as well as progesterone was abolished by pretreatment of the glioma cells with finasteride, an inhibitor of 5alpha-reduced steroid production. Moreover, the stimulatory actions of isoproterenol and serotonin on GFAP gene expression were inhibited by pretreatment with a GABA(A) receptor antagonist bicuculline and a progesterone receptor antagonist RU486. These findings suggest that both adrenergic and serotonergic stimulation may indirectly activate GFAP gene expression probably through the production of 5alpha-reduced steroid metabolites in rat C6 glioma cells, proposing the possibility that 5alpha-reduced neurosteroids may play a potential role in the neuronal regulation of glial cell differentiation.

Androgens, aging, and Alzheimer's disease

Testosterone depletion is a normal consequence of aging in men that is associated with senescent effects in androgen- responsive tissues. We discuss new evidence that one consequence of testosterone depletion in men is an increased risk for the development of Alzheimer's disease (AD). Furthermore, we discuss two candidate mechanisms by which testosterone may affect AD pathogenesis. First, testosterone has been identified as an endogenous regulator of beta-amyloid, a protein that abnormally accumulates in AD brain and is implicated as a causal factor in the disease. Second, findings from several different paradigms indicate that testosterone has both neurotrophic and neuroprotective functions. These new findings support the clinical evaluation of androgen-based therapies for the prevention and treatment of AD.

Nerve growth factor induces elevation of steroid 5alpha-reductase mRNA levels in rat C6 glioma cells through expression of transcription factor Egr-1

Steroid 5alpha-reductase type 1 (5alpha-R), the enzyme converting progesterone and other steroid hormones to their 5alpha-reduced metabolites, has been shown to be localized in both neuronal and glial cells, and this enzyme in glial cells has previously been reported to be activated either by co-culturing with neuronal cells or by adding the conditioned medium of neuronal cells, thus suggesting that neuronal activity may be implicated in the regulation of neurosteroid metabolism in brain. In the present study, to investigate a potential role of neurotrophic factors in the mechanism regulating the production of neuroactive 5alpha-reduced steroid metabolites, the direct action of NGF on 5alpha-R gene expression was examined by measuring the steady-state levels of 5alpha-R mRNA levels in rat C6 glioma cells. Exposure of the glioma cells to NGF increased both 5alpha-R mRNA and its protein levels, and induced the transient elevation of Egr-1 mRNA levels prior to the expression of 5alpha-R mRNA in the cells. Furthermore, NGF failed to induce any significant elevation of 5alpha-R mRNA levels in the cells pretreated with Egr-1 antisense oligodeoxynucleotides. These findings indicate that NGF induces the elevation of 5alpha-R gene expression in the glioma cells through the expression of transcription factor Egr-1, proposing the possibility that NGF, and probably other neurotrophic factors as well, may play a potential role in the regulation of 5alpha-reduced steroid production as one of the factors mediating the intercellular communication between neuronal and glial cells in the brain.

Adrenergic activation of steroid 5alpha-reductase gene expression in rat C6 glioma cells: involvement of cyclic amp/protein kinase A-mediated signaling pathway

Steroid 5alpha-reductase (5alpha-R) is well known as the enzyme converting progesterone and other steroid hormones to their 5alpha-reduced metabolites and has been reported to be localized in both neuronal and glial cells in the brain. Previously, the enzyme activity in glial cells has been shown to be enhanced either by coculturing with neuronal cells or by adding the conditioned medium of neuronal cells, suggesting a possible implication of neuro-glial interactions in the regulation of neurosteroid metabolism in the brain. In the present studies, the effects of adrenergic agonists on 5alpha-R mRNA and protein levels in rat C6 glioma cells were examined as one of the model experiments for investigating the influence of neuronal activity on the expression of 5alpha-R gene in the glial cell. The direct challenge of beta-adrenergic agonists to glioma cells resulted in the rapid and transient elevation of 5alpha-R mRNA levels through the activation of the cyclic AMP (cAMP)/protein kinase A-mediated signaling pathway. Further studies showed that cAMP-induced 5alpha-R mRNA expression was completely abolished by pretreatment of cells with actinomycin D and also indicated that the elevation of 5alpha-R mRNA levels was accompanied by an increase in enzyme protein in the cells. These findings provide strong evidence that the stimulation of beta-adrenergic receptors might induce the transcriptional activation of 5alpha-R gene expression in glial cells, proposing the possibility that neuronal activity might be involved in the production of neuroactive 5alpha-reduced steroids in the brain.

Formation and effects of neuroactive steroids in the central and peripheral nervous system

This chapter summarizes several observations that emphasize the importance of neuroactive steroids in the physiology of the central and peripheral nervous systems. A new, and probably important, concept is emerging: Neuroactive steroids not only modify neuronal physiology but also intervene in the control of glial cell functions. The data presented here underscore that (1) the mechanism of action of the various steroidal molecules may involve both classical (progesterone and androgens) and nonclassical steroid receptors [gamma-aminobutyric acid type A (GABAA) receptor], (2) in many instances, the actions of hormonal steroids are not due to their native molecular forms but to their 5 alpha- and 3 alpha,5 alpha-reduced metabolites, (3) several neuroactive steroids exert dramatic actions on the proteins proper of the peripheral myelin (e.g., glycoprotein Po and peripheral myelin protein 22), and (4) the effects of steroids and of their metabolites might have clinical significance in cases in which the rebuilding of the peripheral myelin is needed (e.g., aging, peripheral injury).

Sex steroids and the brain: lessons from animal studies

Gonadal steroid hormones have multiple effects throughout development on steroid responsive tissues in the brain. The belief that the cellular morphology of the adult brain cannot be modulated or that the synaptic connectivity is "hard-wired" is being rapidly refuted by abundant and growing evidence. Indeed, the brain is capable of undergoing many morphological changes throughout life and gonadal steroids play an important role in many of these processes. Gonadal steroids are implicated in the development of sexually dimorphic structures in the brain, in the control of physiological behaviors and functions and the brain's response to physiological or harmful substances. The effect of sex steroids on neuroprotection and neuroregeneration is an important and expanding area of investigation. Astroglia are targets for estrogen and testosterone and are apparently involved in the actions of sex steroids on the central nervous system. Sex hormones induce changes in the expression of glial fibrillary acidic protein, the growth of astrocytic processes and the extent to which neuronal membranes are covered by astroglial processes. These changes are linked to modifications in the number of synaptic inputs to neurons and suggest that astrocytes may participate in the genesis of gonadal steroid-induced sex differences in synaptic connectivity and synaptic plasticity in the adult brain. Astrocytes and tanycytes may also participate in the cellular effects of sex steroids by releasing neuroactive substances and by regulating the local accumulation of specific growth factors, such as insulin-like growth factor-I, that are involved in estrogen-induced synaptic plasticity and estrogen-mediated neuroendocrine control. Astroglia may also be involved in the regenerative and neuroprotective effects of sex steroids since astroglial activation after brain injury or after peripheral nerve axotomy is regulated by sex hormones.

Influence of serum-free culture conditions on steroid 5alpha-reductase mRNA expression in rat C6 glioma cells

Immunocytochemical studies previously showed that serum deprivation resulted in the appearance of steroid 5alpha-reductase (5alpha-R) in the cytoplasm of rat C6 glioma cells. To determine whether this increase in cytoplasmic 5alpha-R was due to changes in 5alpha-R gene expression, the effect of serum deprivation on 5alpha-R mRNA expression was examined. No significant change in the mRNA levels was observed in cells grown in serum-free culture medium. Therefore, the appearance of 5alpha-R immunoreactivity in the cell cytoplasm observed under serum-free conditions is probably not due to changes in 5alpha-R gene expression.

Regulation of proopiomelanocortin gene expression by endogenous ligands of the GABAA receptor complex as evaluated by in situ hybridization in the rat pars intermedia

The neurotransmitter gamma-aminobutyric acid (GABA) exerts a tonic inhibitory influence on proopiomelanocortin (POMC) neurons in the hypothalamus as well as on the melanotrope cells of the intermediate lobe (IL) of the pituitary gland. Moreover, the activation of the GABAA receptor complex by different ligands has been shown to exert a negative influence on the POMC gene expression at the hypothalamic level. In order to elucidate the in vivo regulation of the POMC mRNA levels in the intermediate lobe of the pituitary by endogenous ligands of the GABAA receptor complex, we have studied the effect of intravenous (i.v.) and intracerebroventricular (i.c.v) injections of octadecaneuropeptide (ODN), a peptide derived from diazepam-binding inhibitor (DBI). The possible involvement of neurosteroids in the action of ODN on melanotropic cells was evaluated following inhibition of two enzymes involved in the biosynthesis of neurosteroids known as activators of G3BAA receptor complex: trilostane, an inhibitor of 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), and MK-906, an inhibitor of 5 alpha-reductase. The i.v. injection of ODN produced a dose-dependent inhibition of POMC gene expression in the IL. The i.c.v. injection of ODN also depressed POMC mRNA. These effects were completely reversed by the concomitant administration of the GABAA antagonist picrotoxin. Similar results were obtained in POMC neurons in the arcuate nucleus (AN) of the hypothalamus. Trilostane administration induced an increase in POMC mRNA and also prevented the inhibitory influence of ODN. The neurosteroid pregnenolone-sulfate, a negative modulator of the GABAA receptor, also stimulated POMC gene expression. On the other hand, MK-906 produced a decrease in mRNA levels and could not reverse the effect of ODN. The results indicate that activation of the GABAA receptor complex by the endogenous benzodiazepine receptor ligand ODN can induce a negative regulation of POMC gene expression in the IL of the pituitary and neurons in the AN. The present results do not provide clear evidence that neurosteroids are involved in the action of ODN on POMC gene expression in the IL.

Testosterone and progesterone metabolism in the central nervous system: cellular localization and mechanism of control of the enzymes involved

This paper summarizes the most recent data obtained in the authors' laboratory on the metabolism of testosterone and progesterone in neurons and in the glia. 1. The activities of 5 alpha-reductase (the enzyme that converts testosterone into dihydrotestosterone; DHT) and of 3 alpha-hydroxy steroid dehydrogenase (the enzyme that converts DHT into 5 alpha-androstane-3 alpha, 17 beta-diol; 3 alpha-diol) were first evaluated in primary cultures of neurons, oligodendrocytes, and type-1 and type-2 astrocytes, obtained from the fetal or neonatal rat brain. The formation of DHT and 3 alpha-diol was evaluated incubating the different cultures with labeled testosterone or labeled DHT as substrates. The results obtained indicate that the formation of DHT takes place preferentially in neurons; however, also type-2 astrocytes and oligodendrocytes possess considerable 5 alpha-reductase activity. A completely different localization was observed for 3 alpha-hydroxysteroid dehydrogenase; the formation of 3 alpha-diol appears to be prevalently, if not exclusively, present in type-1 astrocytes; 3 alpha-diol is formed in very low yields by neurons, type-2 astrocytes, and oligodendrocytes. Moreover, the results indicate that, in type 1 astrocytes, both 5 alpha-reductase and 3 alpha-HSD are stimulated by coculture with neurons and by the addition of neuron-conditioned medium, suggesting that secretory products released by neurons might intervene in the control of glial cell function. 2. Subsequently it was shown that, similarly to what happens when testosterone is used as the substrate, 5 alpha-reductase, which metabolizes progesterone into 5 alpha-pregnane-3,20-dione, (DHP), shows a significantly higher activity in neurons than in glial cells; however, also type-1 and type-2 astrocytes as well as oligodendrocytes possess some ability to 5 alpha-reduce progesterone. On the contrary, 3 alpha-hydroxysteroid dehydrogenase, the enzyme which converts DHP into 5 alpha-pregnane-3 alpha-ol-20-one (THP), appears to be present mainly in type-1 astrocytes; much lower levels of this enzyme are present in neurons and in type-2 astrocytes. At variance with the previous results obtained using androgens as precursors, oligodendrocytes show considerable 3 alpha-hydroxysteroid dehydrogenase activity, even if this is statistically lowe than that present in type-1 astrocytes. The existence of isoenzymatic forms of the enzymes involved in androgen and progesterone metabolism is discussed.

Gonadal steroids and astroglial plasticity

1. Recent evidence indicates that astroglia participate in the metabolism of gonadal hormones, in the synthesis of neurosteroids, and in the plastic responses of neurons to gonadal steroids. The role of astroglia on plastic responses of neural tissue to gonadal hormones and neurosteroids is examined in this review. 2. Gonadal steroids and neurosteroids promote astroglia plasticity in several areas of the central nervous system, including the hypothalamus, the striatum, and the hippocampus. 3. Gonadal steroids and neurosteroids modulate astroglia proliferation and the formation of reactive astroglia after brain injury. 4. Astroglia is a source of trophic factors that may mediate effects of gonadal steroids on neural tissue. 5. Astroglia is involved in the promotion of synaptic plastic changes by gonadal hormones. 6. The effect of gonadal hormones on astroglial plasticity is dependent on specific membrane interactions with neurons and on the expression of the embryonic highly polysialylated isoform of the neural cell adhesion molecule on neuronal membranes. 7. In conclusion, coordinated responses of neurons and astroglia appear to be involved in the modulation of neural function and response to injury by gonadal hormones and neurosteroids.

Sexual dimorphism and sex steroid modulation of glial fibrillary acidic protein messenger RNA and immunoreactivity levels in the rat hypothalamus

By using the techniques of in situ hybridization histochemistry and immunocytochemistry, we have found that both glial fibrillary acidic protein messenger RNA levels and glial fibrillary acidic protein immunoreactive surface density in the arcuate nucleus and median eminence are modulated by both the neonatal and adult sex steroid environments. No effect was seen on the number of immunoreactive glia. Intact adult males had significantly higher glial fibrillary acidic protein messenger RNA levels and glial fibrillary acidic protein immunoreactive surface density than females. Both adult and neonatal castration of male animals significantly reduced glial fibrillary acidic protein messenger RNA levels and glial fibrillary acidic protein immunoreactive surface density. Neonatal and adult testosterone treatment increased both of these parameters in both sexes; however, there was no additive effect of the steroid treatments. Glial cells are involved in the proliferation, survival, migration and maturation of neurons, as well as in the modulation of synaptic connectivity, and therefore it follows that hormonal modulation of glia may mediate some of the known steroid effects on neurons. The data reported here show that astroglia are significantly influenced by both the neonatal and adult sex steroid environments and suggest that some of the steroid effects on neurons during both of these developmental periods may be mediated, at least in part, through modulation of glial cells.

Metabolism of steroids in pure cultures of neurons and glial cells: role of intracellular signalling

In the brain, the 5 alpha-reductase converting testosterone (T) is present both in neurons and in glial cells, even if it prevails in neurons; the 3 alpha-hydroxysteroid-dehydrogenase (3 alpha-HSD), the enzyme converting dihydrotestosterone (DHT) into 3 alpha-diol, is particularly concentrated in type 1 astrocytes. In glial cells, since the 5 alpha-reductase is activated by a cAMP analogue, PKA seems to be involved in the control of this enzyme, postulating that nervous inputs utilizing cAMP as the second messenger might modify the activity of this enzyme in glial cells. Moreover, the results indicate that, in type 1 astrocytes, both the 5 alpha-reductase and the 3 alpha-HSD are stimulated by the co-culture with neurons and by the addition of neuron-conditioned medium, suggesting that secretory products released by neurons might intervene in the control of glial cell function.

Gonadal hormone regulation of neuronal-glial interactions in the developing neuroendocrine hypothalamus

Recent evidence indicates that, in addition to their well known effects on neurons, gonadal steroids may exert part of their neural effects through astroglia. In adult female rats astroglia participate in the phasic remodelling of synapses that takes place during the estrous cycle in the arcuate nucleus of the hypothalamus under the influence of estradiol. Astroglia also appear to be involved in the genesis of sex differences in synaptic connectivity. Gonadal steroids influence hypothalamic astroglia differentiation in vitro and in vivo. In monolayer mixed neuronal-glial cultures from fetal rat hypothalami, estradiol induces a progressive differentiation of astrocytes from a flattened epithelioid morphology to bipolar, radial and stellate shapes. This effect of estradiol on astroglia is dependent on the expression of specific molecules on the neuronal surface, such as the polysialic acid-rich form of the neural cell adhesion molecule. In the rat arcuate nucleus in situ, perinatal androgen influences astroglia gene expression and differentiation, resulting in a sex difference in astroglia organization by postnatal day 20. By this day, the amount of neuronal surface covered by astroglial processes is higher in males than in females. This difference in the coverage of neuronal surface by astroglia may be directly related to the reduced number of synaptic contacts that is established on the soma of male neurons compared to females.

Gonadal hormones as promoters of structural synaptic plasticity: cellular mechanisms

It is now obvious that the CNS is capable of undergoing a variety of plastic changes at all stages of development. Although the magnitude and distribution of these changes may be more dramatic in the immature animal, the adult brain retains a remarkable capacity for undergoing morphological and functional modifications. Throughout development, as well as in the postpubertal animal, gonadal steroids exert an important influence over the architecture of specific sex steroid-responsive areas, resulting in sexual dimorphisms at both morphological and physiological levels. We are only now beginning to gain insight into the mechanisms involved in gonadal steroid-induced synaptic changes. The number of synaptic inputs to specific neuronal populations is sexually dimorphic and this can be modulated by changes in the sex steroid environment. These modifications can be correlated with other morphological changes, such as glial cell activation, that are occurring simultaneously in the same anatomical area. Indeed, the close physical relationship between glial cells and neuronal synaptic contacts makes them an ideal candidate for participating in this process. Interestingly, not only can the morphology and immunoreactivity of glial cells be modulated by gonadal steroids, but a close negative correlation between the number of synapses and the amount of glial ensheathing of a neuron has been demonstrated, suggesting an active participation of these cells in this process. Glia have sex steroid receptors, are capable of producing and metabolizing steroids, and can produce other neuronal trophic factors in response to sex steroids. Hence, their role in gonadal steroid-induced synaptic plasticity is becoming more apparent. In addition, there is recent evidence that this process may involve certain cell surface molecules, such as the N-CAMs, since a specific isoform of this molecule, previously referred to as the embryonic form, is found in those areas of the brain which maintain the capacity to undergo synaptic remodelling. However, there is much work to be done in order to fully understand this phenomenon and before bringing it into a clinical setting in hopes of treating neurodegenerative diseases or injuries to the nervous system.

Gonadal steroids as promoters of neuro-glial plasticity

Estradiol induces coordinated modifications in the extension of glial and neuronal processes in the arcuate nucleus of the hypothalamus of adult female rats. This hormonal effect results in natural fluctuations in the ensheathing of arcuate neurons by glial processes and these glial changes are linked to a remodelling of inhibitory GABAergic synapses during the estrous cycle. Hormonally induced glial and synaptic changes appear to be dependent on specific recognition or adhesion molecules on the neuronal and/or glial membranes.

Androgen and progesterone metabolism in the central and peripheral nervous system

This paper summarizes the most recent data obtained in the authors' laboratory on the metabolism of testosterone and progesterone in neurons, in the glia, and in neuroblastoma cells. The activities of the 5 alpha-reductase (the enzyme that converts testosterone into dihydrotestosterone, DHT), and of the 3 alpha-hydroxysteroid dehydrogenase (the enzyme that converts DHT into 5 alpha-androstane-3 alpha, 17 beta-diol, 3 alpha-diol) have been first evaluated in primary cultures of neurons, oligodendrocytes and type-1 and -2 astrocytes, obtained from the fetal or neonatal rat brain. All the cultures were used on the fifth day. The formation of DHT of 3 alpha-diol was evaluated incubating the different cultures with labeled testosterone or DHT as substrates. The results obtained indicate that the formation of DHT takes place preferentially in neurons; however, type-2 astrocytes and oligodendrocytes also possess considerable 5 alpha-reductase activity, while type-1 astrocytes show a much lower enzymatic concentration. A completely different localization was observed for 3 alpha-hydroxysteroid dehydrogenase; the formation of 3 alpha-diol appears to be prevalently, if not exclusively, present in type-1 astrocytes; 3 alpha-diol is formed in very low yields by neurons, type-2 astrocytes and oligodendrocytes. The compartmentalization of two strictly correlated enzymes (5 alpha-reductase and 3 alpha-hydroxysteroid dehydrogenase) in separate central nervous system (CNS) cell populations suggests the simultaneous participation of neurons and glial cells in the 5 alpha-reductive metabolism of testosterone. Subsequently it has been shown that, similarly to what happens when testosterone is used as the substrate, the 5 alpha-reductase which metabolizes progesterone into 5 alpha-pregnane-3,20-dione (DHP) shows a significantly higher activity in neurons than in glial cells; however, type-1 and -2 astrocytes as well as oligodendrocytes also possess some ability to 5 alpha-reduce progesterone. On the other hand, 3 alpha-hydroxysteroid dehydrogenase, the enzyme which converts DHP into 5 alpha-pregnane-3 alpha-ol-20-one, appears to be present mainly in type-1 astrocytes; much lower levels of this enzyme are present in neurons and in type-2 astrocytes. At variance with the previous results obtained using androgens as precursors, oligodendrocytes show considerable 3 alpha-hydroxysteroid dehydrogenase activity, even if this is statistically lower than that present in type-1 astrocytes. The existence of isoforms of the enzyme involved in androgen and progesterone metabolism is discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Testosterone and progesterone metabolism in the human neuroblastoma cell line SH-SY5Y

The ability of the human neuroblastoma cell line SH-SY5Y to metabolize androgens and progesterone was studied by incubating the cells in the presence of labeled testosterone (T) or progesterone (P) to measure, respectively, the formation of dihydrotestosterone (DHT) or dihydroprogesterone (DHP) (5 alpha-reductase activity). The 3 alpha-hydroxysteroid dehydrogenase activity was studied by evaluating the conversion of labeled DHT into 5 alpha-androstan-3 alpha, 17 beta-diol (3 alpha-diol). The results show that undifferentiated neuroblastoma cells possess a significant 5 alpha-reductase activity, as shown by the considerable conversion of T into DHT; moreover, this enzymatic activity seems to be significantly stimulated following cell differentiation induced by the phorbol ester TPA, but not after differentiation induced by retinoic acid (RA). The 5 alpha-reductase(s) present in SH-SY5Y cells is also able to convert P into DHP. In undifferentiated cells, this conversion was about 8 times higher than that of T into DHT. Under the influences of TPA and RA, the formation of DHP followed the same pattern observed for the formation of DHT. SH-SY5Y cells also appear to possess the enzyme 3 alpha-hydroxysteroid dehydrogenase, since they are able to convert DHT into 3 alpha-diol. This enzymatic activity is not altered following TPA-induced differentiation and appears to be decreased following treatment with RA. It is suggested that the SH-SY5Y cell line may represent a useful "in vitro" model for the study of the mechanisms involved in the control of androgen and P metabolism in nervous cells.

Gonadal hormones down-regulate reactive gliosis and astrocyte proliferation after a penetrating brain injury

Astrocytes are a target for gonadal steroids in the normal brain. The putative modulation by gonadal hormones of the astrocytic reaction to brain injury was assessed in this study. Male and female adult Wistar albino rats were gonadectomized and, one month later, their brains were lesioned by a longitudinal incision crossing the parietal cerebral cortex, the CA1 field of the dorsal hippocampus and the dentate gyrus. Males were injected either with testosterone (20 micrograms/rat) or vehicle immediately after surgery. Females were injected either with 17 beta estradiol (250 micrograms/rat), progesterone (500 micrograms/rat) or vehicle. Hormonal injections were repeated 24 and 48 h after brain injury. All animals received injections of 5'-bromodeoxyuridine (BrdU) to label proliferating cells. Histological sections from the brain of animals killed 72 h after surgery were used for the double immunohistochemical localization of BrdU and glial fibrillary acidic protein (GFAP). The number of GFAP-immunoreactive astrocytes and the number of double labelled astrocytes (GFAP + BrdU) were recorded as a function of the distance to the lesion site in the parietal cerebral cortex, the CA1 field of the hippocampus and the dentate gyrus. Testosterone, estradiol and progesterone treatments resulted in a significant decrease in the number of GFAP-immunolabeled reactive astrocytes in the vicinity of the wound. The number of double labelled cells and the labelling index (proportion of GFAP-immunoreactive astrocytes labelled with BrdU) varied according to the cerebral area, the distance to the wound and the sex of the animals, and were significantly decreased by gonadal steroids in all the areas examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Natural fluctuation and gonadal hormone regulation of astrocyte immunoreactivity in dentate gyrus

The number and the surface density of cells immunoreactive for the specific astrocytic marker glial fibrillary acidic protein (GFAP), were evaluated in both the hilus of the dentate gyrus and the granular layer of the vermis of the cerebellar cortex of adult female rats during the different phases of the estrous cycle, after ovariectomy and after the pharmacological administration of estradiol and/or progesterone to ovariectomized rats. Although no significant differences were detected in the number of immunoreactive cells among the different experimental groups studied, their surface density showed significant changes in the hilus of the dentate gyrus. The surface density of immunoreactive cells was increased in the afternoon of proestrus and on the morning of estrus compared to the morning of proestrus, diestrus, and metestrus, was decreased after ovariectomy, and showed a dose-dependent increase in ovariectomized rats injected with 17 beta estradiol (1, 10, or 300 micrograms/rat), alone or in combination with progesterone (500 micrograms/rat). In contrast, it was not affected by the administration of 17 alpha estradiol (300 micrograms/rat). The surface density of immunoreactive cells was significantly increased over control values by 5 h after the injection of 17 beta estradiol (300 micrograms/rat) and as early as 1 h after the administration of progesterone. The separate injection of either 17 beta estradiol or progesterone had smaller effects on the surface density of immunoreactive cells than did the administration of both hormones together. The surface density of GFAP-immunoreactive cells reached maximal values by 24 h after the administration of 17 beta estradiol and/or progesterone and returned to control levels by 48 h after the combined injection of progesterone and 17 beta estradiol, while in the rats that were injected with only one of the two hormones, the surface density of immunoreactive cells remained over control values for at least 9 days. No such hormonal effects on GFAP-immunoreactive cells were observed in the cerebellar cortex.