Estrogen mediation of injury-induced cell birth in neuroproliferative regions of the adult zebra finch brain

Chondroitin sulfate proteoglycans mRNA expression and degradation in the zebra finch following traumatic brain injury

Traumatic brain injury (TBI) is one of the leading causes of fatality and disability worldwide. From minutes to months following damage, injury can result in a complex pathophysiology that can lead to temporary or permanent deficits including an array of neurodegenerative symptoms. These changes can include behavioral dysregulation, memory dysfunctions, and mood changes including depression. The nature and severity of impairments resulting from TBIs vary widely given the range of injury type, location, and extent of brain tissue involved. In response to the injury, the brain induces structural and functional changes to promote repair and minimize injury size. Despite its high prevalence, effective treatment strategies for TBI are limited. PNNs are part of the neuronal extracellular matrix (ECM) that mediate synaptic stabilization in the adult brain and thus neuroplasticity. They are associated mostly with inhibitory GABAergic interneurons and are thought to be responsible for maintaining the excitatory/inhibitory balance of the brain. The major structural components of PNNs include multiple chondroitin sulfate proteoglycans (CSPGs) as well as other structural proteins. Here we examine the effects of injury on CSPG expression, specifically around the changes in the side change moieties. To investigate CSPG expression following injury, adult male and female zebra finches received either a bilateral penetrating, or no injury and qPCR analysis and immunohistochemistry for components of the CSPGs were examined at 1- or 7-days post-injury. Next, to determine if CSPGs and thus PNNs should be a target for therapeutic intervention, CSPG side chains were degraded at the time of injury with chondroitinase ABC (ChABC) CSPGs moieties were examined. Additionally, GABA receptor mRNA and aromatase mRNA expression was quantified following CSPG degradation as they have been implicated in neuronal survival and neurogenesis. Our data indicate the CSPG moieties change following injury, potentially allowing for a brief period of synaptic reorganization, and that treatments that target CSPG side chains are successful in further targeting this brief critical period by decreasing GABA mRNA receptor expression, but also decreasing aromatase expression.

Neuroestradiol and neuronal development: Not an exclusive male tale anymore

The brain synthesizes a variety of neurosteroids, including neuroestradiol. Inhibition of neuroestradiol synthesis results in alterations in basic neurodevelopmental processes, such as neurogenesis, neuroblast migration, neuritogenesis and synaptogenesis. Although the neurodevelopmental actions of neuroestradiol are exerted in both sexes, some of them are sex-specific, such as the well characterized effects of neuroestradiol derived from the metabolism of testicular testosterone during critical periods of male brain development. In addition, recent findings have shown sex-specific actions of neuroestradiol on neuroblast migration, neuritic growth and synaptogenesis in females. Among other factors, the epigenetic regulation exerted by X linked genes, such as Kdm6a/Utx, may determine sex-specific actions of neuroestradiol in the female brain. This review evidences the impact of neuroestradiol on brain formation in both sexes and highlights the interaction of neural steriodogenesis, hormones and sex chromosomes in sex-specific brain development.

Sex Steroids, Adult Neurogenesis, and Inflammation in CNS Homeostasis, Degeneration, and Repair

Sex steroidal hormones coordinate the development and maintenance of tissue architecture in many organs, including the central nervous systems (CNS). Within the CNS, sex steroids regulate the morphology, physiology, and behavior of a wide variety of neural cells including, but not limited to, neurons, glia, endothelial cells, and immune cells. Sex steroids spatially and temporally control distinct molecular networks, that, in turn modulate neural activity, synaptic plasticity, growth factor expression and function, nutrient exchange, cellular proliferation, and apoptosis. Over the last several decades, it has become increasingly evident that sex steroids, often in conjunction with neuroinflammation, have profound impact on the occurrence and severity of neuropsychiatric and neurodegenerative disorders. Here, I review the foundational discoveries that established the regulatory role of sex steroids in the CNS and highlight recent advances toward elucidating the complex interaction between sex steroids, neuroinflammation, and CNS regeneration through adult neurogenesis. The majority of recent work has focused on neuroinflammatory responses following acute physical damage, chronic degeneration, or pharmacological insult. Few studies directly assess the role of immune cells in regulating adult neurogenesis under healthy, homeostatic conditions. As such, I also introduce tractable, non-traditional models for examining the role of neuroimmune cells in natural neuronal turnover, seasonal plasticity of neural circuits, and extreme CNS regeneration.

Central Administration of Indomethacin Mitigates the Injury-Induced Upregulation of Aromatase Expression and Estradiol Content in the Zebra Finch Brain

Injury to the vertebrate brain causes neuroinflammation, characterized in part by increases in prostaglandins. In rodents and songbirds, brain injury also induces the transcription and translation of aromatase in reactive astrocytes around the site of damage. Interestingly, this induction is more rapid in female zebra finches relative to males. Induced aromatization is neuroprotective, as inhibition of aromatase and estrogen replacement, increases and decreases the extent of damage, respectively. Although the consequences of induced astrocytic aromatization are intensely studied, little is known about what factors induce aromatase. Inflammation is sufficient to induce astrocytic aromatase suggesting that the link between inflammation and aromatase expression may be causal. To test this hypothesis, adult male and female zebra finches received bilateral mechanical injuries through which either the cyclooxygenase (COX)-1/2 inhibitor indomethacin or vehicle was administered into contralateral hemispheres. Subjects were killed either 6 or 24 hours after injury. In both sexes, an enzyme immunoassay for prostaglandin E2 (PGE2) revealed that indomethacin decreased PGE2 relative to the contralateral hemisphere at both time points, suggesting that the dose and mode of administration used were successful in affecting neuroinflammation locally. Indomethacin reduced aromatase expression and 17β-estradiol (E2) content at 6 hours but not 24 hours following injury in females. However, in males, the inhibitory effect of indomethacin on aromatase and E2 was apparent at 24 but not 6 hours after treatment. These data suggest that COX activity, perhaps via consequent prostaglandin secretion, may induce aromatase expression and central E2, an effect that is detectable in temporally distinct patterns between sexes.

A Quantification of the Injury-Induced Changes in Central Aromatase, Oestrogenic Milieu and Steroid Receptor Expression in the Zebra Finch

In songbirds and mammals, brain injury results in the up-regulation of aromatase (oestrogen synthase) expression in astroglia. The resulting presumed synthesis of neural oestradiol (E2 ) has neuroprotective effects including a decrease in neurodegeneration, neuroinflammation and apoptosis. The development of therapeutic tools that exploit oestrogenic neuroprotection in the treatment of neurotrauma requires a precise quantification of the endogenous changes in neural aromatase and E2 following brain injury. Surprisingly, the expected increase in neural oestrogens following brain injury has not been demonstrated. Furthermore, we are just beginning to unravel the mechanisms behind the protective effects of centrally synthesised E2 . In the present study, levels of aromatase immunoprotein, neural E2 and steroid receptor mRNA were quantified in adult male and female zebra finches 48 h following a unilateral penetrating brain injury. Both aromatase and E2 were up-regulated in the injured hemisphere of the brain compared to the uninjured hemisphere, demonstrating for the first time a robust increase in neural E2 levels following injury. We did not detect an effect of injury on mRNA expression of the oestrogen receptors (ER)-α, ER-β or GPER-1, but observed a significant decrease in androgen receptor transcription in the injured lobe relative to the contralateral uninjured hemisphere. We conclude that mechanical damage causes a dramatic increase in local aromatisation, and the resultant high levels of central E2 are available to modulate steroid sensitive targets. Studies using alternate methods of receptor detection and/or time points may be necessary to understand the complete suite of mechanisms underlying the neuroprotective effects of induced oestrogen synthesis in this animal model.

Estradiol treatment decreases cell proliferation in the neurogenic zones of adult female zebrafish (Danio rerio) brain

While estrogens are known to play a crucial role in the neurogenesis of the mammalian and avian brain, their role in teleost adult proliferation pattern is not yet fully understood. The present study aimed to determine the estrogen effects in adult brain proliferation zones, using zebrafish, as a model organism. Indeed, teleost fish brain provides a unique adult neurogenesis model, based on its extensive proliferation, contrasting the restricted adult telencephalic neurogenesis observed in birds and mammals. To determine the effect of estrogens, 17-β estradiol was administrated for 7 days in adult female zebrafish, followed by bromodeoxyuridine (BrdU)-immunohistochemistry and double immunofluorescence. Stereological analyses of the BrdU-positive cells within the neurogenic zones, showed region-specific decreases of actively proliferating cells in the estrogen-treated animals, compared to matched controls. Interestingly, the most prominent estradiol effects were found in the number of cycling cells of the ventral nucleus of ventral telencephalic area (Vv) and cerebellar areas. Significant decreases were also determined in the dorso-lateral telencephalic, preoptic and dorsal hypothalamic areas. In contrast, medial dorsal telencephalic, caudal (Hc) and ventral (Hv) hypothalamic areas were unaffected by estrogen treatment. The majority of the BrdU-labeled cells were found to co-express PCNA proliferating marker in Hc, Hv and Vv. Additionally, a population of proliferating cells co-expressed the early neuronal marker TOAD in all areas studied. These results provide significant evidence on the 17-β estradiol impact on adult neurogenesis, down-regulating the fast-cycling and post-mitotic cells within the female zebrafish brain neurogenetic zones.

Acute and chronic effects of an aromatase inhibitor on pair-maintenance behavior of water-restricted zebra finch pairs

Zebra finches are highly social songbirds that maintain life-long monogamous pair-bonds. They rely heavily upon these pair-bonds to survive their ever-changing and unpredictable habitat in the Australian desert. These pair-bonds are maintained via a large repertoire of affiliative behaviors that for most of an individual's life are predominately associated with pair maintenance. Water restriction reduces circulating testosterone levels in male zebra finches and the size of the ovary and oviduct in female zebra finches, but water restriction has little or no effects on pair-maintenance behaviors and local levels of testosterone and estradiol in behaviorally-relevant brain regions. These data suggest that in water-restricted zebra finches, local synthesis of testosterone and estradiol in the brain may support the expression of pair-maintenance behaviors. Here, we directly test whether pair-maintenance behaviors are regulated by estradiol, acting via non-genomic or genomic mechanisms, in water-restricted (i.e., non-breeding) zebra finches. In two experiments, subjects were treated with an aromatase inhibitor (fadrozole) either acutely or chronically, and a variety of pair-maintenance behaviors were quantified. Additionally, we quantified the effect of acute fadrozole treatment on brain and circulating estradiol and testosterone levels. Acute fadrozole administration rapidly decreased estradiol levels in the circulation and brain of males and also rapidly increased testosterone levels in the circulation and brain of both males and females. However, neither the acute nor chronic fadrozole treatment decreased pair-maintenance behaviors. In one case, acute fadrozole treatment promoted affiliation. These data suggest that pair-maintenance behavior in non-breeding zebra finches is not promoted by estradiol acting via either non-genomic or genomic mechanisms.

Induced synthesis of oestrogens by glia in the songbird brain

Studies on birds have long provided landmarks and touchstones in the fields of neuroendocrinology, immunology and neuroplasticity. The passerine brain is an excellent model for studying the actions of hormones, including steroids, on a diversity of behavioural endpoints. Oestrogens, for example, have profound effects on avian neuroanatomy and neurophysiology throughout life and, importantly, are synthesised at high levels within neurones of the songbird brain. More recently, aromatisation in another set of neural cells has been identified. Specifically, aromatase expression is induced in astrocytes and radial glia following disruption of the neuropil by multiple forms of perturbation. The avian brain, therefore, can be provided with high levels of oestrogens constitutively or via induction, by aromatisation in neurones and glia, respectively. In this review, we begin with the initial discovery of aromatisation by non-neuronal cells and discuss the mechanisms underlying the induction of aromatase expression in glial cells. We then focus on the emerging interactions between the neuroendocrine and neuroimmune systems with respect to brain injury. Next, we briefly review the extensive literature on the influence of glial aromatisation on neuroplasticity, and end with some recent data on sex differences in the induction of glial aromatase in the zebra finch. Throughout this review, we consider the unanswered questions and future studies that may emerge from these findings.

Traumatized and inflamed--but resilient: glial aromatization and the avian brain

Steroids like estrogens have potent effects on the vertebrate brain, and are provided to neural targets from peripheral and central sources. Estradiol synthesized within the vertebrate CNS modulates neural structure and function, including the pathways involved in neuroprotection, and perhaps, neural repair. Specifically, aromatase; the enzyme responsible for the conversion of testosterone to estradiol, is upregulated in the avian and mammalian brain following disruption of the neuropil by multiple forms of perturbation including mechanical injury, ischemia and excitotoxicity. This injury induced aromatase expression is somewhat unique in that it occurs in astroglia rather than neurons, and is stimulated in response to factors associated with brain damage. In this review, we focus on the induction, expression and consequences of glial aromatization in the songbird brain. We begin with a review of the anatomical consequences of glial estrogen provision followed by a discussion of the cellular mechanisms whereby glial aromatization may affect injury-induced neuroplasticity. We then present the current status of our understanding regarding the inductive role of inflammatory processes in the transcription and translation of astrocytic aromatase. We consider the functional aspects of glial aromatization before concluding with unanswered questions and suggestions for future studies. Birds have long informed us about fundamental questions in endocrinology, immunology, and neuroplasticity; and their unique anatomical and physiological characteristics continue to provide an excellent system in which to learn about brain trauma, inflammation, and neuroprotection.

Birds as a model to study adult neurogenesis: bridging evolutionary, comparative and neuroethological approaches

During the last few decades, evidence has demonstrated that adult neurogenesis is a well-preserved feature throughout the animal kingdom. In birds, ongoing neuronal addition occurs rather broadly, to a number of brain regions. This review describes adult avian neurogenesis and neuronal recruitment, discusses factors that regulate these processes, and touches upon the question of their genetic control. Several attributes make birds an extremely advantageous model to study neurogenesis. First, song learning exhibits seasonal variation that is associated with seasonal variation in neuronal turnover in some song control brain nuclei, which seems to be regulated via adult neurogenesis. Second, food-caching birds naturally use memory-dependent behavior in learning the locations of thousands of food caches scattered over their home ranges. In comparison with other birds, food-caching species have relatively enlarged hippocampi with more neurons and intense neurogenesis, which appears to be related to spatial learning. Finally, migratory behavior and naturally occurring social systems in birds also provide opportunities to investigate neurogenesis. This diversity of naturally occurring memory-based behaviors, combined with the fact that birds can be studied both in the wild and in the laboratory, make them ideal for investigation of neural processes underlying learning. This can be done by using various approaches, from evolutionary and comparative to neuroethological and molecular. Finally, we connect the avian arena to a broader view by providing a brief comparative and evolutionary overview of adult neurogenesis and by discussing the possible functional role of the new neurons. We conclude by indicating future directions and possible medical applications.

Environmental enrichment rescues postnatal neurogenesis defect in the male and female Ts65Dn mouse model of Down syndrome

Down syndrome (DS), the most frequent genetic cause of intellectual disability and developmental delay, results from impaired neural stem cell proliferation and differentiation. Impaired neurogenesis in the neocortex, hippocampus and cerebellum is believed to be the underlying cause of learning and behavioral deficits in the Ts65Dn mouse model of DS. Aggressive sensorimotor and cognitive therapies have shown promise in mitigating the cognitive disabilities in DS but these behavioral therapies have not yet been investigated at the cellular level. Here, using the Ts65Dn mouse model of DS, we demonstrate that a combination of environmental enrichment and physical exercise starting in juvenile mice (postnatal day 18) markedly increases cell proliferation, neurogenesis and gliogenesis in the hippocampal dentate gyrus (DG) and the forebrain subventricular zone (SVZ) of both male and female mice. Enrichment and exercise increased the rate of Ts65Dn DG neurogenesis to be comparable to that of the nonenriched euploid group, while the effect on SVZ neurogenesis was reduced and seen only after prolonged exposure. These results clearly indicate that in a comprehensive stimulatory environment, the postnatal DS brain has the intrinsic capability of improving neurogenesis and gliogenesis to the levels of normal matched controls and that this cellular response underlies the cognitive improvement seen following behavioral therapies.

Estrogenic regulation of dopaminergic neurons in the opportunistically breeding zebra finch

Steroid-induced changes in dopaminergic activity underlie many correlations between gonadal hormones and social behaviors. However, the effects of steroid hormones on the various behaviorally relevant dopamine cell groups remain unclear, and ecologically relevant species differences remain virtually unexplored. We examined the effects of estradiol (E2) manipulations on dopamine (DA) neurons of male and female zebra finches (Taeniopygia guttata), focusing on numbers of tyrosine hydroxylase-immunoreactive (TH-ir) cells in the A8-A15 cell groups, and on TH colocalization with Fos, conducted in the early A.M., in order to quantify basal transcriptional activity. TH is the rate-limiting enzyme for catecholamine synthesis, and specifically DA in the A8-A15 cell groups. In contrast to other examined birds and mammals, reducing E2 levels with the aromatase-inhibitor Letrozole failed to alter TH-ir neuron numbers within the ventral tegmental area (VTA; A10), while increasing neuron numbers in the central gray (CG; A11) and caudal midbrain A8 populations. Consistent with findings in other birds, but not mammals, we also found no effects of E2 manipulations (Letrozole or Letrozole plus E2 replacement) on TH-Fos colocalization in any location. In accordance with previous observations in both mammals and birds, E2 treatment decreased the number of TH-ir neurons in the A12 population of the tuberal hypothalamus, a cell group that inhibits the release of prolactin. In general, males and females exhibited similar TH-ir neuron numbers, although males exhibited significantly more TH-ir neurons in the A11 CG population than did females. These results suggest partial variability in E2 regulation of DA across species.

Newborn GnRH neurons in the adult forebrain of the ring dove

The preoptic area of the hypothalamus is a key area that produces gonadotrophin-releasing hormone (GnRH). In birds, the chicken GnRH-I-form neurons are responsible for the hypothalamus-pituitary-gonadal system, which controls reproduction. In the ring dove, electrolytic lesion in the adult hypothalamus induces neurogenesis. In this study, we determined whether adult neurogenesis is involved in repairing GnRH neurons, specifically by generating newborn cells exhibiting GnRH-I immunoreactive properties. We selectively applied electrolytic lesions to three different regions of the diencephalon, including the preoptic area, which contains GnRH-I neurons, and identified new cells (BrdU-positive cells) that co-labeled with GnRH-I-immunoreactive cells. The BrdU(+)/GnRH(+) double labeled cells were then confirmed with confocal laser analysis. In brains of both male and female ring doves we found new neurons at the lesion site of the preoptic region that were GnRH-I immunoreactive. However, the total number of GnRH neurons in the lesioned brains was less than that of sham-lesioned brains. When two other regions of the diencephalon that contain GnRH-I neurons were damaged, no recruitment of new GnRH-I neurons was detected. The rate of neurogenesis depends on the bird's reproductive phase when the lesion was applied. We found BrdU(+)/GnRH(+) double-labeled cells almost exclusively during the pre-laying phase when birds are engaged in active courtship that leads to egg laying. Our observations suggest that recruitment of GnRH immunoreactive new neurons is restricted to the hypothalamic region and is sensitive to the reproductive stage of the birds.

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

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

Steroidal and gonadal effects on neural cell proliferation in vitro in an adult songbird

Neurogenesis in the adult songbird brain occurs along the ventricular zone (VZ), a specialized cell layer surrounding the lateral ventricles. To examine the acute effects of sex steroids on VZ cell proliferation, male and female adult zebra finch brain slices containing the VZ were exposed to 5-bromo-2'-deoxyuridine-5'-monophosphate (BrdU) in vitro. Slices from one hemisphere served as the control, while contralateral slices were treated with steroids, steroidogenic enzyme inhibitors or gonadal tissue itself. There were no significant effects on VZ cell proliferation in either sexes by acute exposure to 17beta-estradiol (E2), dihydrotestosterone (DHT), a cocktail of four sex steroids, and inhibitors of sex steroid synthesis (aminoglutethimide, ketoconazole, and fadrozole), or by activation of a mitochondrial cholesterol transporter. By contrast, dehydroepiandrosterone (DHEA) suppressed VZ cell proliferation in males, but not females, replicating previous observations involving treatments with corticosterone and RU-486. This suggests that DHEA suppresses proliferation in males via a glucocorticoid receptor-related mechanism. These results suggest that neurosteroidogenesis per se has little effect on acute VZ cell proliferation. Co-incubation with an ovary of female, but not male, slices significantly increased VZ cell proliferation; testicular tissue had no impact on proliferation in males or females. This suggests a role for a non-steroidal ovarian factor on adult female VZ cell proliferation. We also have evidence that previously reported sex-differences in BrdU-labeling along the adult VZ (males>females) result from a more rapid loss of cells in females. Sex differences in steroid action and cell death along the VZ may contribute to the maintenance of the sexually dimorphic song system.

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

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

Cryptic regulation of vasotocin neuronal activity but not anatomy by sex steroids and social stimuli in opportunistic desert finches

In most vertebrate species, the production of vasotocin (VT; non-mammals) and vasopressin (VP; mammals) in the medial bed nucleus of the stria terminalis (BSTm) waxes and wanes with seasonal reproductive state; however, opportunistically breeding species might need to maintain high levels of this behaviorally relevant neuropeptide year-round in anticipation of unpredictable breeding opportunities. We here provide support for this hypothesis and demonstrate that these neurons are instead regulated 'cryptically' via hormonal regulation of their activity levels, which may be rapidly modified to adjust VT signaling. First, we show that combined treatment of male and female zebra finches (Estrildidae: Taeniopygia guttata) with the androgen receptor antagonist flutamide and the aromatase inhibitor 1,4,6-androstatriene-3,17-dione does not alter the expression of VT immunoreactivity within the BSTm; however, both hormonal treatment and social housing environment (same-sex versus mixed-sex) alter VT colocalization with the immediate early gene product Fos (a proxy marker of neural activation) in the BSTm. In a second experiment, manipulations of estradiol (E2) levels with the aromatase inhibitor letrozole (LET) or subcutaneous E2 implants failed to alter colocalization, suggesting that the colocalization effects in experiment 1 were solely androgenic. LET treatment also did not affect VT immunoreactivity in a manner reversible by E2 treatment. Finally, comparisons of VT immunoreactivity in breeding and nonbreeding individuals of several estrildid species demonstrate that year-round stability of VT immunoreactivity is found only in highly opportunistic species, and is therefore not essential to the maintenance of long-term pair bonds, which are ubiquitous in the Estrildidae.

Species-specific injury-induced cell proliferation in the hippocampus and subventricular zone of food-storing and nonstoring wild birds

Cells are continuously born and incorporated into the adult hippocampus (HP). Adult neurogenesis might act to increase the total number of cells or replace dead cells. Thus, neurogenesis might be a primary factor in augmenting, maintaining, or even recovering functions. In zebra finches, HP injury increases cell proliferation in the HP and stem cell rich subventricular zone (SVZ). It is unknown what effect injury has on a species dependent upon the HP for survival in the wild. In food-storing birds, recovery of caches is seasonal, necessary for survival, dependent upon the HP and is concomitant with a peak in HP neurogenesis. During the fall, food-storing black-capped chickadees (BCCs) and nonstoring dark-eyed juncos (DEJs) were captured and given a unilateral penetrating lesion to the HP one day later. On day 3, birds were injected with the mitotic marker 5-bromo-2'-deoxyuridine (BrdU) and perfused on day 10. If unlesioned, more BrdU-labeled cells were observed in the HP and SVZ of BCCs compared to DEJs, indicating higher innate cell proliferation or incorporation in BCCs. If lesioned, BrdU-labeled cells increased in the injured HP of both species; however, lesions caused larger increases in DEJs. DEJs also showed increases in BrdU-labeled cells in the SVZ and contralateral HP. BCCs showed no such increases on day 10. Thus, during the fall food-storing season, storers showed suppressed injury-induced cell proliferation and/or reduced survival rates of these new cells compared to nonstorers. These species differences may provide a useful model for isolating factors involved in cellular responses following injury.

Therapeutic implications of brain steroidogenesis

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

Interplay of hormones and p53 in modulating gender dimorphism of subventricular zone cell number

Previous studies have suggested the existence of a gender bias in repair after demyelination. Here we report the existence of gender dimorphism for the regulation of cell number in the subventricular zone (SVZ), an area that has been studied for its repair potential. The number of Sox2(+) multipotential cells in the SVZ of young adult female mice was greater than in age-matched male siblings, but this difference was not evident prior to the surge of sex hormones (i.e., in prepubertal mice). To begin asking whether hormonally derived signals were responsible for these gender-related differences, we analyzed proliferation and survival of cultured male- and female-derived SVZ cells. Estrogen, but not testosterone treatment increased cell proliferation and survival of cultured cells after IFN-gamma treatment or after UV irradiation, regardless of the gender of origin. Because apoptosis in UV-irradiated SVZ cells correlated with the expression of the proapoptotic molecule p53, we postulated that this molecule could be responsible for the gender dimorphism in the SVZ. In agreement with this prediction, no difference in the SVZ cell number was detected in male and female p53 null mice. Together with previous reports, these results implicate p53 as an important component of the mechanism regulating gender dimorphism in the SVZ.

Neurosteroid production in the songbird brain: a re-evaluation of core principles

Concepts of brain-steroid signaling have traditionally placed emphasis on the gonads and adrenals as the source of steroids, the strict dichotomy of early developmental ("organizational") and mature ("activational") effects, and a relatively slow mechanism of signaling through intranuclear receptors. Continuing research shows that these concepts are not inaccurate, but they are certainly incomplete. In this review, we focus on the song control circuit of songbird species to demonstrate how each of these concepts is limited. We discuss the solid evidence for steroid synthesis within the brain ("neurosteroidogenesis"), the role of neurosteroids in organizational events that occur both early in development and later in life, and how neurosteroids can act in acute and non-traditional ways. The songbird model therefore illustrates how neurosteroids can dramatically increase the diversity of steroid-sensitive brain functions in a behaviorally-relevant system. We hope this inspires further research and thought into neurosteroid signaling in songbirds and other animals.

Neuroprotective actions of brain aromatase

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

Recovery of motor and cognitive function after cerebellar lesions in a songbird: role of estrogens

In addition to its key role in complex motor function, the cerebellum is increasingly recognized to have a role in cognition. Songbirds are particularly good models for the investigation of motor and cognitive processes but little is known about the role of the songbird cerebellum in these processes. To explore cerebellar function in a songbird, we lesioned the cerebellum of adult female zebra finches and examined the effects on a spatial working memory task and on motor function during this task. There is evidence for steroid synthesis in the songbird brain and neurosteroids may have an impact on some forms of neural plasticity in adult songbirds. We therefore hypothesized that neurosteroids would affect motor and cognitive function after a cerebellar injury. We found that cerebellar lesions produced deficits in motor and cognitive aspects of a spatial task. In line with our prediction, birds in which estrogen synthesis was blocked had impaired performance in our spatial task compared with those that had estrogen synthesis blocked but estrogen replaced. There was no clear effect of estrogen replacement on motor function. We also found that lesions induced expression of the estrogen synthetic enzyme aromatase in reactive astrocytes and Bergmann glia around a cerebellar lesion. These data suggest that the cerebellum of songbirds mediates both motor and cognitive function and that estrogens may improve the recovery of cognitive aspects of cerebellar function after injury.

Sex differences in cell proliferation and glucocorticoid responsiveness in the zebra finch brain

Neural proliferation is a conserved property of the adult vertebrate brain. In mammals, stress reduces hippocampal neuronal proliferation and the effect is stronger in males than in females. We tested the effects of glucocorticoids on ventricular zone cell proliferation in adult zebra finches where neurons are produced that migrate to and incorporate within the neural circuits controlling song learning and performance. Adult male zebra finches sing and have an enlarged song circuitry; females do not sing and the song circuit is poorly developed. Freshly prepared slices from adult males and females containing the lateral ventricles were incubated with the mitotic marker BrdU with or without steroid treatments. BrdU-labeled cells were revealed immunocytochemically and all labeled cells within the ventricular zone were counted. We identified significantly higher rates of proliferation along the ventricular zone of males than in females. Moreover, acute administration of corticosterone significantly reduced proliferation in males with no effects in females. This effect in males was replicated by RU-486, which appears to act as an agonist of the glucocorticoid receptor in the songbird brain. The corticosterone effect was reversed by Thiram, which disrupts corticosterone action on the glucocorticoid receptor. Sex differences in proliferation and responses to stress hormones may contribute to the sexually dimorphic and seasonal growth of the neural song system of songbirds.

Glial aromatization increases the expression of bone morphogenetic protein-2 in the injured zebra finch brain

In songbirds, brain injury upregulates glial aromatase. The resulting local estrogen synthesis mitigates apoptosis and enhances cytogenesis by poorly understood mechanisms. Bone morphogenetic proteins (BMPs), long studied for their role in neural development, are also neuroprotective and cytogenic in the adult brain. BMPs remain uncharacterized in songbirds, as do the mechanisms regulating their post-injury expression. We first established the expression of BMPs 2, 4, 6, and 7 in the adult zebra finch brain using RT-PCR. Next, we determined the effect of neural insult on BMP expression, by comparing BMP transcripts between injured and uninjured telencephalic hemispheres using semi-quantitative PCR. The expression of BMPs 2 and 4, but not 6 and 7, increased 24 h post-injury. To determine the influence of aromatase on BMP expression, we compared BMP expression following delivery of the aromatase inhibitor Fadrozole or vehicle into contralateral hemispheres. Fadrozole decreased BMP2, but not BMP4, expression, suggesting that aromatization may induce BMP2 expression following injury. Since BMPs are gliogenic and neurotrophic, future studies will test if the neuroprotective and cytogenic effects of aromatase upregulation are mediated by BMP2. Songbirds may be excellent models towards understanding the role of local estrogen synthesis and its downstream mechanisms on neuroprotection and repair.

Aromatase in the brain: not just for reproduction anymore

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

Aromatase expression and cell proliferation following injury of the adult zebra finch hippocampus

Estrogens can be neuroprotective following traumatic brain injury. Immediately after trauma to the zebra finch hippocampus, the estrogen-synthetic enzyme aromatase is rapidly upregulated in astrocytes and radial glia around the lesion site. Brain injury also induces high levels of cell proliferation. Estrogens promote neuronal differentiation, migration, and survival naturally in the avian brain. We suspect that glia are a source of estrogens promoting cell proliferation after neural injury. To explore this hypothesis, we examined the spatial and temporal relationship between glial aromatase expression and cell proliferation after neural injury in adult female zebra finches. Birds were ovariectomized and given a blank implant or one filled with estradiol; some birds were also administered an aromatase inhibitor or vehicle. All birds received penetrating injuries to the right hippocampus. Twenty-four hours after lesioning, birds were injected once with BrdU to label mitotically active cells and euthanized 2 h, 24 h, or 7 days later. The brains were processed for double-label BrdU and aromatase immunocytochemistry. Injury-induced glial aromatase expression was unaffected by survival time and aromatase inhibition. BrdU labeling was significantly reduced at 24 h by ovariectomy and by aromatase inhibition; effects were partially reversed by E2 replacement. Irrespective of ovariectomy, the densities of aromatase immunoreactive astrocytes and BrdU-labeled cells at known distances from the lesion site were highly correlated. These data suggest that injury-induced glial aromatization may influence the reorganization of injured tissue by providing a rich estrogenic environment available to influence cellular incorporation.