Cloning of the zebra finch androgen synthetic enzyme CYP17: a study of its neural expression throughout posthatch development

Tracing the development of learned song preferences in the female zebra finch brain with functional magnetic resonance imaging

In sexually dimorphic zebra finches (Taeniopygia guttata), only males learn to sing their father's song, whereas females learn to recognize the songs of their father or mate but cannot sing themselves. Memory of learned songs is behaviorally expressed in females by preferring familiar songs over unfamiliar ones. Auditory association regions such as the caudomedial mesopallium (CMM; or caudal mesopallium) have been shown to be key nodes in a network that supports preferences for learned songs in adult females. However, much less is known about how song preferences develop during the sensitive period of learning in juvenile female zebra finches. In this study, we used blood-oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to trace the development of a memory-based preference for the father's song in female zebra finches. Using BOLD fMRI, we found that only in adult female zebra finches with a preference for learned song over novel conspecific song, neural selectivity for the father's song was localized in the thalamus (dorsolateral nucleus of the medial thalamus; part of the anterior forebrain pathway, AFP) and in CMM. These brain regions also showed a selective response in juvenile female zebra finches, although activation was less prominent. These data reveal that neural responses in CMM, and perhaps also in the AFP, are shaped during development to support behavioral preferences for learned songs.

Effect of a Territorial Challenge on the Steroid Profile of a Juvenile Songbird

Aggression is a social behavior that is critical for survival and reproduction. In adults, circulating gonadal hormones, such as androgens, act on neural circuits to modulate aggressive interactions, especially in reproductive contexts. In many species, individuals also demonstrate aggression before reaching gonadal maturation. Adult male song sparrows, Melospiza melodia, breed seasonally but maintain territories year-round. Juvenile (hatch-year) males aggressively compete for territory ownership during their first winter when circulating testosterone is low. Here, we characterized the relationship between the steroid milieu and aggressive behavior in free-living juvenile male song sparrows in winter. We investigated the effect of a 10 min simulated territorial intrusion (STI) on behavior and steroid levels in blood, 10 microdissected brain regions, and four peripheral tissues (liver, pectoral muscle, adrenal glands, and testes). Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), we quantified 12 steroids: pregnenolone, progesterone, corticosterone, 11-dehydrocorticosterone, dehydroepiandrosterone, androstenedione, testosterone, 5α-dihydrotestosterone, 17β-estradiol, 17α-estradiol, estrone, and estriol. We found that juvenile males are robustly aggressive, like adult males. An STI increases progesterone and corticosterone levels in blood and brain and increases 11-dehydrocorticosterone levels in blood only. Pregnenolone, androgens, and estrogens are generally non-detectable and are not affected by an STI. In peripheral tissues, steroid concentrations are very high in the adrenals. These data suggest that adrenal steroids, such as progesterone and corticosterone, might promote juvenile aggression and that juvenile and adult songbirds might rely on distinct neuroendocrine mechanisms to support similar aggressive behaviors.

The form, function, and evolutionary significance of neural aromatization

Songbirds have emerged as exceptional research subjects for helping us appreciate and understand estrogen synthesis and function in brain. In the context of recognizing the vertebrate-wide importance of brain aromatase expression, in this review we highlight where we believe studies of songbirds have provided clarification and conceptual insight. We follow by focusing on more recent studies of aromatase and neuroestrogen function in the hippocampus and the pallial auditory processing region NCM of songbirds. With perspectives drawn from this body of work, we speculate that the evolution of enhanced neural estrogen signaling, including in the mediation of social behaviors, may have given songbirds the resilience to radiate into one of the most successful vertebrate groups on the planet.

Profiling of systemic and brain steroids in male songbirds: Seasonal changes in neurosteroids

Steroids are secreted by the gonads and adrenal glands into the blood to modulate neurophysiology and behaviour. In addition, the brain can metabolise circulating steroids and synthesise steroids de novo. Songbirds show high levels of neurosteroid synthesis. In the present study, we developed and validated a liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay for the measurement of 10 steroids in whole blood, plasma and microdissected brain tissue (1-2 mg) of song sparrows. Our assay is highly accurate, precise, specific and sensitive. Moreover, the liquid-liquid extraction is fast, simple and effective. We quantified steroids in the blood and brain of wild male song sparrows in both breeding and non-breeding seasons. As expected, systemic androgen levels were higher in the breeding season than in the non-breeding season. Brain androgens were detectable only in the breeding season; androstenedione and 5α-dihydrotestosterone levels were up to 20-fold higher in specific brain regions than in blood. Oestrogens were not detectable in blood in both seasons. Oestrone and 17β-oestradiol were detectable in brain in the breeding season only (up to 1.4 ng g^-1 combined). Progesterone levels in several regions were higher in the non-breeding season than the breeding season, despite the lack of seasonal changes in systemic progesterone. Corticosterone levels in the blood were higher in the breeding season than in the non-breeding season but showed few seasonal differences in the brain. In general, the steroid levels presented here are lower than those in previous reports using immunoassays, because of the higher specificity of mass spectrometry. We conclude that (i) brain steroid levels can differ greatly from circulating steroid levels and (ii) brain steroid levels show region-specific seasonal patterns that are not a simple reflection of circulating steroid levels. This approach using ultrasensitive LC-MS/MS is broadly applicable to other species and allows steroid profiling in microdissected brain regions.

Testosterone synthesis in the female songbird brain

Decades of work have established the brain as a source of steroid hormones, termed 'neurosteroids'. The neurosteroid neuroestradiol is produced in discrete brain areas and influences cognition, sensory processing, reproduction, neurotransmission, and disease. A prevailing research focus on neuroestradiol has essentially ignored whether its immediate synthesis precursor - the androgen testosterone - is also dynamically regulated within the brain. Testosterone itself can rapidly influence neurophysiology and behavior, and there is indirect evidence that the female brain may synthesize significant quantities of testosterone to regulate cognition, reproduction, and behavior. In songbirds, acoustic communication is regulated by neuroestrogens. Neuroestrogens are rapidly synthetized in the caudomedial nidopallium (NCM) of the auditory cortex of zebra finches in response to song and can influence auditory processing and song discrimination. Here, we examined the in vivo dynamics of NCM levels of the neuroestrogen synthesis precursor, testosterone. Unlike estradiol, testosterone did not appear to fluctuate in the female NCM during song exposure. However, a substantial song-induced elevation of testosterone was revealed in the left hemisphere NCM of females when local aromatization (i.e., conversion to estrogens) was locally blocked. This elevation was eliminated when local androgen synthesis was concomitantly blocked. Further, no parallel elevation was observed in the circulation in response to song playback, consistent with a local, neural origin of testosterone synthesis. To our knowledge, this study provides the first direct demonstration that testosterone fluctuates rapidly in the brain in response to socially-relevant environmental stimuli. Our findings suggest therefore that locally-derived 'neuroandrogens' can dynamically influence brain function and behavior. SIGNIFICANCE STATEMENT: This study demonstrates that androgen synthesis occurs rapidly in vivo in the brain in response to social cues, in a lateralized manner. Specifically, testosterone synthesis occurs within the left secondary auditory cortex when female zebra finches hear male song. Therefore, testosterone could act as a neuromodulator to rapidly shape sensory processing. Androgens have been linked to functions such as the control of female libido, and many steroidal drugs used for contraception, anti-cancer treatments, and sexual dysfunction likely influence the brain synthesis and action of testosterone. The current findings therefore establish a clear role for androgen synthesis in the female brain with implications for understanding neural circuit function and behavior in animals, including humans.

Inhibitory cell populations depend on age, sex, and prior experience across a neural network for Critical Period learning

In many ways, the complement of cell subtypes determines the information processing that a local brain circuit can perform. For example, the balance of excitatory and inhibitory (E/I) signaling within a brain region contributes to response magnitude and specificity in ways that influence the effectiveness of information processing. An extreme example of response changes to sensory information occur across Critical Periods (CPs). In primary mammalian visual cortex, GAD65 and parvalbumin inhibitory cell types in particular control experience-dependent responses during a CP. Here, we test how the density of GAD65- and parvalbumin-expressing cells may inform on a CP for complex behavioral learning. Juvenile male zebra finch songbirds (females cannot sing) learn to sing through coordinated sensory, sensorimotor, and motor learning processes distributed throughout a well-defined neural network. There is a CP for sensory learning, the process by which a young male forms a memory of his “tutor’s” song, which is then used to guide the young bird’s emerging song structure. We quantified the effect of sex and experience with a tutor on the cell densities of GAD65- and parvalbumin-expressing cells across major nodes of the song network, using ages that span the CP for tutor song memorization. As a resource, we also include whole-brain mapping data for both genes. Results indicate that inhibitory cell populations differ across sex, age, and experiential conditions, but not always in the ways we predicted.

Kobayashi award: Discovery of cerebellar and pineal neurosteroids and their biological actions on the growth and survival of Purkinje cells during development (review)

The brain has traditionally been considered to be a target site of peripheral steroid hormones. On the other hand, extensive studies over the past thirty years have demonstrated that the brain is a site of biosynthesis of several steroids. Such steroids synthesized de novo from cholesterol in the brain are called neurosteroids. To investigate the biosynthesis and biological actions of neurosteroids in the brain, data on the regio- and temporal-specific synthesis of neurosteroids are needed. In the mid 1990s, the Purkinje cell, an important cerebellar neuron, was discovered as a major cell producing neurosteroids in the brain of vertebrates. It was the first demonstration of de novo neuronal biosynthesis of neurosteroids in the brain. Subsequently, neuronal biosynthesis of neurosteroids and biological actions of neurosteroids have become clear by the follow-up studies using the Purkinje cell as an excellent cellular model. Progesterone and estradiol, which are known as sex steroid hormones, are actively synthesized de novo from cholesterol in the Purkinje cell during development, when cerebellar neuronal circuit formation occurs. Importantly, progesterone and estradiol synthesized in the Purkinje cell promote dendritic growth, spinogenesis and synaptogenesis via their cognate nuclear receptors in the Purkinje cell. Neurotrophic factors may mediate these neurosteroid actions. Futhermore, allopregnanolone (3α,5α-tetrahydroprogesterone), a progesterone metabolite, is also synthesized in the cerebellum and acts on the survival of Purkinje cells. On the other hand, at the beginning of 2010s, the pineal gland, an endocrine organ located close to the cerebellum, was discovered as an important site of the biosynthesis of neurosteroids. Allopregnanolone, a major pineal neurosteroid, acts on the Purkinje cell for the survival of Purkinje cells by suppressing the expression of caspase-3, a crucial mediator of apoptosis. I as a recipient of Kobayashi Award from the Japan Society for Comparative Endocrinology in 2016 summarize the discovery of cerebellar and pineal neurosteroids and their biological actions on the growth and survival of Purkinje cells during development.

A chromosomal inversion predicts the expression of sex steroid-related genes in a species with alternative behavioral phenotypes

In white-throated sparrows, a chromosomal rearrangement has led to alternative phenotypes that differ in sex steroid-dependent behaviors. The rearrangement has captured the genes estrogen receptor alpha and 5-alpha reductase, making these genes strong candidates for mediating the behavioral phenotypes. We report here that of the two genes, expression of estrogen receptor alpha mRNA differs between the morphs and predicts behavior to a much greater extent than does expression of 5-alpha reductase mRNA. Differentiation of estrogen receptor alpha, therefore, is likely more important for the behavioral phenotypes. We also found that in some brain regions, the degree to which testosterone treatment affects the expression of steroid-related genes depends strongly on morph. A large morph difference in estrogen receptor alpha mRNA expression in the amygdala appears to be independent of plasma testosterone; this difference persists during the non-breeding season and is detectable in nestlings at post-hatch day seven. The latter result suggests a substrate for organizational effects of hormones during development.

Seasonal regulation of steroidogenic enzyme expression within the green anole lizard (Anolis carolinensis) brain and gonad

Steroid hormones, such as testosterone and estradiol, are necessary for reproductive behavior. Seasonally breeding animals have increased sex steroid hormone levels during the breeding compared to non-breeding season, with increased reproductive behaviors and altered brain morphology in breeding individuals. Similar to other seasonally breeding animals, green anole lizards (Anolis carolinensis) have high sex steroid hormone levels and increased reproductive behaviors in the breeding season. Relatively less is known regarding the regulation of steroidogenesis in reptiles and this experiment examined whether enzymes involved in sex steroid hormone synthesis vary seasonally within the brain and gonads in wild-caught anole lizards. Specifically, we examined mRNA expression of steroidogenic acute regulatory protein (StAR), P450 17α-hydroxylase/C17-20lyase (Cyp17α1), 17 beta-hydroxysteroid dehydrogenase type 3 (17βHSD 3), and aromatase (Cyp19α1). We found that the mRNA for each of these genes was expressed in the lizard brain. Interestingly, Cyp19α1 mRNA expression in the brain was increased during the non-breeding season, potentially revealing a role for aromatase expression in the non-breeding brain. In the anole gonads, StAR mRNA expression levels were increased in both males and females during the breeding season, while the mRNA expression levels of CYP17α1 and 17βHSD 3 are increased when StAR mRNA expression was decreased, suggesting that the enzymes in the steroidogenic pathway are potentially regulated independently of StAR. This work reveals the seasonal regulation of steroidogenesis in the reptilian brain and gonad, although more work is necessary to determine the regulatory mechanisms that control these expression patterns.

7α-Hydroxypregnenolone regulating locomotor behavior identified in the brain and pineal gland across vertebrates

The brain synthesizes steroids de novo from cholesterol, which are called neurosteroids. Based on extensive studies on neurosteroids over the past thirty years, it is now accepted that neurosteroidogenesis in the brain is a conserved property across vertebrates. However, the formation of bioactive neurosteroids in the brain is still incompletely elucidated in vertebrates. In fact, we recently identified 7α-hydroxypregnenolone (7α-OH PREG) as a novel bioactive neurosteroid stimulating locomotor behavior in the brain of several vertebrates. The follow-up studies have demonstrated that the stimulatory action of brain 7α-OH PREG on locomotor behavior is mediated by the dopaminergic system across vertebrates. More recently, we have further demonstrated that the pineal gland, an endocrine organ located close to the brain, is a major site of the formation of bioactive neurosteroids. In addition to the brain, the pineal gland actively produces 7α-OH PREG de novo from cholesterol as a major pineal neurosteroid that acts on the brain to control locomotor rhythms. This review summarizes the identification, biosynthesis and mode of action of brain and pineal 7α-OH PREG, a new bioactive neurosteroid regulating locomotor behavior, across vertebrates.

Developmental song learning as a model to understand neural mechanisms that limit and promote the ability to learn

Songbirds famously learn their vocalizations. Some species can learn continuously, others seasonally, and still others just once. The zebra finch (Taeniopygia guttata) learns to sing during a single developmental "Critical Period," a restricted phase during which a specific experience has profound and permanent effects on brain function and behavioral patterns. The zebra finch can therefore provide fundamental insight into features that promote and limit the ability to acquire complex learned behaviors. For example, what properties permit the brain to come "on-line" for learning? How does experience become encoded to prevent future learning? What features define the brain in receptive compared to closed learning states? This piece will focus on epigenomic, genomic, and molecular levels of analysis that operate on the timescales of development and complex behavioral learning. Existing data will be discussed as they relate to Critical Period learning, and strategies for future studies to more directly address these questions will be considered. Birdsong learning is a powerful model for advancing knowledge of the biological intersections of maturation and experience. Lessons from its study not only have implications for understanding developmental song learning, but also broader questions of learning potential and the enduring effects of early life experience on neural systems and behavior.

A reliable and flexible gene manipulation strategy in posthatch zebra finch brain

Songbird models meaningfully contribute to many fields including learned vocal communication, the neurobiology of social interactions, brain development, and ecology. The value of investigating gene-brain-behavior relationships in songbirds is therefore high. Viral infections typically used in other lab animals to deliver gene editing constructs have been less effective in songbirds, likely due to immune system properties. We therefore leveraged the in vivo electroporation strategy used in utero in rodents and in ovo in poultry, and apply it to posthatch zebra finch songbird chicks. We present a series of experiments with a combination of promoters, fluorescent protein genes, and piggyBac transposase vectors to demonstrate that this can be a reliable, efficient, and flexible strategy for genome manipulation. We discuss options for gene delivery experiments to test circuit and behavioral hypotheses using a variety of manipulations, including gene overexpression, CRISPR/Cas9 gene editing, inducible technologies, optogenetic or DREADD cellular control, and cell type-specific expression.

Non-ovarian aromatization is required to activate female sexual motivation in testosterone-treated ovariectomized quail

Although aromatase is expressed in both male and female brains, its functional significance in females remains poorly understood. In female quail, sexual receptivity is activated by estrogens. However it is not known whether sexual motivation is similarly estrogen-dependent and whether estrogens locally produced in the brain contribute to these behavioral responses. Four main experiments were designed to address these questions. In Experiment 1 chronic treatment of females with the anti-estrogen tamoxifen decreased their receptivity, confirming that this response is under the control of estrogens. In Experiment 2 chronic treatment with tamoxifen significantly decreased sexual motivation as treated females no longer approached a sexual partner. In Experiment 3 (a) ovariectomy (OVX) induced a significant decrease of time spent near the male and a significantly decreased receptivity compared to gonadally intact females, (b) treatment with testosterone (OVX+T) partially restored these responses and (c) this effect of T was prevented when estradiol synthesis was inhibited by the potent aromatase inhibitor Vorozole (OVX+T+VOR). Serum estradiol concentration was significantly higher in OVX+T than in OVX or OVX+T+VOR females. Together these data demonstrate that treatment of OVX females with T increases sexual motivation and that these effects are mediated at least in part by non-gonadal aromatization of the androgen. Finally, assays of aromatase activity on brain and peripheral tissues (Experiment 4) strongly suggest that brain aromatization contributes to behavioral effects observed here following T treatment but alternative sources of estrogens (e.g. liver) should also be considered.

How to contribute to the progress of neuroendocrinology: New insights from discovering novel neuropeptides and neurosteroids regulating pituitary and brain functions

Obtaining new insights by discovering novel neuropeptides and neurosteroids regulating pituitary and brain functions is essential for the progress of neuroendocrinology. At the beginning of 1970s, gonadotropin-releasing hormone (GnRH) was discovered in mammals. Since then, it was generally accepted that GnRH is the only hypothalamic neuropeptide regulating gonadotropin release in vertebrates. In 2000, however, gonadotropin-inhibitory hormone (GnIH), a novel hypothalamic neuropeptide that actively inhibits gonadotropin release, was discovered in quail. The follow-up studies demonstrated that GnIH acts as a new key player for regulation of reproduction across vertebrates. It now appears that GnIH acts on the pituitary and the brain to serve a number of behavioral and physiological functions. On the other hand, a new concept has been established that the brain synthesizes steroids, called neurosteroids. The formation of neurosteroids in the brain was originally demonstrated in mammals and subsequently in other vertebrates. Recently, 7α-hydroxypregnenolone was discovered as a novel bioactive neurosteroid inducing locomotor behavior of vertebrates, indicating that neurosteroidogenesis in the brain is still incompletely elucidated in vertebrates. At the beginning of 2010s, it was further found that the pineal gland actively produces neurosteroids. Pineal neurosteroids act on the brain to regulate locomotor rhythms and neuronal survival. Furthermore, the interaction of neuropeptides and neurosteroids is becoming clear. GnIH decreases aggressive behavior by regulating neuroestrogen synthesis in the brain. This review summarizes these new insights by discovering novel neuropeptides and neurosteroids in the field of neuroendocrinology.

Sex differences and rapid estrogen signaling: A look at songbird audition

The actions of estrogens have been associated with brain differentiation and sexual dimorphism in a wide range of vertebrates. Here we consider the actions of brain-derived 'neuroestrogens' in the forebrain and the accompanying differences and similarities observed between males and females in a variety of species. We summarize recent evidence showing that baseline and fluctuating levels of neuroestrogens within the auditory forebrain of male and female zebra finches are largely similar, and that neuroestrogens enhance auditory representations in both sexes. With a comparative perspective we review evidence that non-genomic mechanisms of neuroestrogen actions are sexually differentiated, and we propose a working model for nonclassical estrogen signaling via the MAPK intracellular signaling cascade in the songbird auditory forebrain that is informed by the way sex differences may be compensated. This view may lead to a more comprehensive understanding of how sex influences estradiol-dependent modulation of sensorimotor representations.

Dynamic gene expression in the song system of zebra finches during the song learning period

The brain circuitry that controls song learning and production undergoes marked changes in morphology and connectivity during the song learning period in juvenile zebra finches, in parallel to the acquisition, practice and refinement of song. Yet, the genetic programs and timing of regulatory change that establish the neuronal connectivity and plasticity during this critical learning period remain largely undetermined. To address this question, we used in situ hybridization to compare the expression patterns of a set of 30 known robust molecular markers of HVC and/or area X, major telencephalic song nuclei, between adult and juvenile male zebra finches at different ages during development (20, 35, 50 days post-hatch, dph). We found that several of the genes examined undergo substantial changes in expression within HVC or its surrounds, and/or in other song nuclei. They fit into broad patterns of regulation, including those whose expression within HVC during this period increases (COL12A1, COL 21A1, MPZL1, PVALB, and CXCR7) or decreases (e.g., KCNT2, SAP30L), as well as some that show decreased expression in the surrounding tissue with little change within song nuclei (e.g. SV2B, TAC1). These results reveal a broad range of molecular changes that occur in the song system in concert with the song learning period. Some of the genes and pathways identified are potential modulators of the developmental changes associated with the emergence of the adult properties of the song control system, and/or the acquisition of learned vocalizations in songbirds.

Breakthrough in neuroendocrinology by discovering novel neuropeptides and neurosteroids: 2. Discovery of neurosteroids and pineal neurosteroids

Bargmann-Scharrer's discovery of "neurosecretion" in the first half of the 20th century has since matured into the scientific discipline of neuroendocrinology. Identification of novel neurohormones, such as neuropeptides and neurosteroids, is essential for the progress of neuroendocrinology. Our studies over the past two decades have significantly broadened the horizons of this field of research by identifying novel neuropeptides and neurosteroids in vertebrates that have opened new lines of scientific investigation in neuroendocrinology. We have established de novo synthesis and functions of neurosteroids in the brain of various vertebrates. Recently, we discovered 7α-hydroxypregnenolone (7α-OH PREG), a novel bioactive neurosteroid that acts as a key regulator for inducing locomotor behavior by means of the dopaminergic system. We further discovered that the pineal gland, an endocrine organ located close to the brain, is an important site of production of neurosteroids de novo from cholesterol (CHOL). The pineal gland secretes 7α-OH PREG and 3α,5α-tetrahydroprogesterone (3α,5α-THP; allopregnanolone) that are involved in locomotor rhythms and neuronal survival, respectively. Subsequently, we have demonstrated their mode of action and functional significance. This review summarizes the discovery of these novel neurosteroids and its contribution to the progress of neuroendocrinology.

Establishing regional specificity of neuroestrogen action

The specificity of estrogen signaling in brain is defined at one level by the types and distributions of receptor molecules that are activated by estrogens. At another level, as our understanding of the neurobiology of the estrogen synthetic enzyme aromatase has grown, questions have emerged as to how neuroactive estrogens reach specific target receptors in functionally relevant concentrations. Here we explore the spatial specificity of neuroestrogen signaling with a focus on studies of songbirds to provide perspective on some as-yet unresolved questions. Studies conducted in both male and female songbirds have helped to clarify these interesting facets of neuroestrogen physiology.

Expression of androgen receptor in the brain of a sub-oscine bird with an elaborate courtship display

Sex steroids control vertebrate behavior by modulating neural circuits specialized for sex steroid sensitivity. In birds, receptors for androgens (AR) and estrogens (ERα) show conserved expression in neural circuits controlling copulatory and vocal behaviors. Male golden-collared manakins have become a model for evaluating hormonal control of complex physical courtship displays. These birds perform visually and acoustically elaborate displays involving considerable neuromuscular coordination. Androgens activate manakin courtship and AR are expressed widely in spinal circuits and peripheral muscles utilized in courtship. Using in situ hybridization, we report here the distributions of AR and ERα mRNA in the brains of golden-collared manakins. Overall patterns of AR and ERα mRNA expression resemble what has been observed in non-vocal learning species. Notably, however, we detected a large area of AR expression in the arcopallium, a forebrain region that contains a crucial premotor song nucleus in vocal learning species. These results support the idea that AR signaling both centrally and peripherally is responsible for the activation of male manakin courtship, and the arcopallium is likely a premotor site for AR-mediated displays.

Biosynthesis and biological action of pineal allopregnanolone

The pineal gland transduces photoperiodic changes to the neuroendocrine system by rhythmic secretion of melatonin. We recently provided new evidence that the pineal gland is a major neurosteroidogenic organ and actively produces a variety of neurosteroids de novo from cholesterol in birds. Notably, allopregnanolone is a major pineal neurosteroid that is far more actively produced in the pineal gland than the brain and secreted by the pineal gland in juvenile birds. Subsequently, we have demonstrated the biological action of pineal allopregnanolone on Purkinje cells in the cerebellum during development in juvenile birds. Pinealectomy (Px) induces apoptosis of Purkinje cells, whereas allopregnanolone administration to Px chicks prevents cell death. Furthermore, Px increases the number of Purkinje cells that express active caspase-3, a crucial mediator of apoptosis, and allopregnanolone administration to Px chicks decreases the number of Purkinje cells expressing active caspase-3. It thus appears that pineal allopregnanolone prevents cell death of Purkinje cells by suppressing the activity of caspase-3 during development. This paper highlights new aspects of the biosynthesis and biological action of pineal allopregnanolone.

Advancing avian behavioral neuroendocrinology through genomics

Genome technologies are transforming all areas of biology, including the study of hormones, brain and behavior. Annotated reference genome assemblies are rapidly being produced for many avian species. Here we briefly review the basic concepts and tools used in genomics. We then consider how these are informing the study of avian behavioral neuroendocrinology, focusing in particular on lessons from the study of songbirds. We discuss the impact of having a complete "parts list" for an organism; the transformational potential of studying large sets of genes at once instead one gene at a time; the growing recognition that environmental and behavioral signals trigger massive shifts in gene expression in the brain; and the prospects for using comparative genomics to uncover the genetic roots of behavioral variation. Throughout, we identify promising new directions for bolstering the application of genomic information to further advance the study of avian brain and behavior.

Genome-brain-behavior interdependencies as a framework to understand hormone effects on learned behavior

Hormones have profound effects on the maturation and function of the zebra finch song system. Hormones often signal through receptors that directly or indirectly regulate transcription. In this way, hormones and the genome are functionally connected. Genome-brain-behavior interdependencies are often studied on evolutionary timescales but we can now apply and test these relationships on short timescales, relevant to an individual. Here, we begin to place patterns of hormone-related gene expression into the timeframe of an individual's lifespan to consider how hormones contribute to organization of neural systems necessary for learned behavior, and how they might signal during experience in ways that affect future behavior. This framework illustrates both how much investigations into genome and hormone function are intertwined, and how much we still need to learn.

Brain and pineal 7α-hydroxypregnenolone stimulating locomotor activity: identification, mode of action and regulation of biosynthesis

Biologically active steroids synthesized in the central and peripheral nervous systems are termed neurosteroids. However, the biosynthetic pathways leading to the formation of neurosteroids are still incompletely elucidated. 7α-Hydroxypregnenolone, a novel bioactive neurosteroid stimulating locomotor activity, has been recently identified in the brain of newts and quail. Subsequently, the mode of action and regulation of biosynthesis of 7α-hydroxypregnenolone have been determined. Moreover, recent studies on birds have demonstrated that the pineal gland, an endocrine organ located close to the brain, is an important site of production of neurosteroids de novo from cholesterol. 7α-Hydroxypregnenolone is a major pineal neurosteroid that stimulates locomotor activity in juvenile chickens, connecting light-induced gene expression with locomotion. This review summarizes the advances in our understanding of the identification, mode of action and regulation of biosynthesis of brain and pineal 7α-hydroxypregnenolone, a potent stimulator of locomotor activity.

Hormones and the neuromuscular control of courtship in the golden-collared manakin (Manacus vitellinus)

Many animals engage in spectacular courtship displays, likely recruiting specialized neural, hormonal and muscular systems to facilitate these performances. Male golden-collared manakins (Manacus vitellinus) of Panamanian rainforests perform physically elaborate courtship displays that include novel forms of visual and acoustic signaling. We study the behavioral neuroendocrinology of this male's courtship, combining field behavioral observations with anatomical, biochemical and molecular laboratory-based studies. Seasonally, male courtship is activated by testosterone with little correspondence between testosterone levels and display intensity. Females prefer males whose displays are exceptionally frequent, fast and accurate. The activation of androgen receptors (AR) is crucial for optimal display performance, with AR expressed at elevated levels in several neuromuscular tissues. Apparently, courtship enlists an elaborate androgen-dependent network that includes spinal motoneurons, skeletal muscles and somatosensory systems. This work highlights the value of studying non-traditional species to illuminate physiological adaptations and, hopefully, stimulates future research on other species with complex behaviors.

New biosynthesis and biological actions of avian neurosteroids

De novo neurosteroidogenesis from cholesterol occurs in the brain of various avian species. However, the biosynthetic pathways leading to the formation of neurosteroids are still not completely elucidated. We have recently found that the avian brain produces 7α-hydroxypregnenolone, a novel bioactive neurosteroid that stimulates locomotor activity. Until recently, it was believed that neurosteroids are produced in neurons and glial cells in the central and peripheral nervous systems. However, our recent studies on birds have demonstrated that the pineal gland, an endocrine organ located close to the brain, is an important site of production of neurosteroids de novo from cholesterol. 7α-Hydroxypregnenolone is a major pineal neurosteroid that stimulates locomotor activity of juvenile birds, connecting light-induced gene expression with locomotion. The other major pineal neurosteroid allopregnanolone is involved in Purkinje cell survival during development. This paper highlights new aspects of neurosteroid synthesis and actions in birds.

Calbindin expression in developing striatum of zebra finches and its relation to the formation of area X

A sexually dimorphic network of brain regions controls learning and production of song in zebra finches. How this specialized song system evolved is unknown. To start addressing this question, we focused on developmental differences between the sexes, using the expression of the calcium-binding protein calbindin (CB) during embryonic to adult stages to map out the early development of Area X, a male-specific striatal structure. We related this pattern to the expression of three transcription factors, Pax6 and Islet1 to delineate the striatal radial domains, and Nkx2.1 as a marker for cells of pallidal origin. An incipient Area X-CB+ domain became discernable at embryonic day 13 in the Islet1-ventral striatal field. This region contained many Nkx2.1-expressing cells with a morphology characteristic of migrating cells. Eight days after hatching (PHD) CB staining clearly delineated Area X. Another CB+ structure formed around PHD5 at the subpallial/pallial boundary. We call it the CB+striatal capsule (CB-StC) and discuss its relation with the previously described striatal capsule in vertebrates. The CB cell population in both Area X and CB-StC, but not in the surrounding striatum, colocalized with the striatal medium spiny neurons (MSNs) marker, D1-receptor associated signaling protein dopamine-and-cAMP-regulated phosphoprotein of 32 kDa, DARPP32. In females, CB-positive cells were also present in the rostral striatum but did not coalesce into an Area X-like structure. We discuss possible reasons for CB expression in MSNs in songbirds and mammals, but not described in chicken striatum.

Biosynthesis and biological actions of pineal neurosteroids in domestic birds

The central and peripheral nervous systems have the capacity of synthesizing steroids de novo from cholesterol, the so-called 'neurosteroids'. De novo synthesis of neurosteroids from cholesterol appears to be a conserved property across the subphylum vertebrata. Until recently, it was generally believed that neurosteroids are produced in neurons and glial cells in the central and peripheral nervous systems. However, our recent studies on birds have demonstrated that the pineal gland, an endocrine organ located close to the brain, is an important site of production of neurosteroids de novo from cholesterol. 7α-Hydroxypregnenolone is a major pineal neurosteroid that stimulates locomotor activity of juvenile birds, connecting light-induced gene expression with locomotion. The other major pineal neurosteroid allopregnanolone is involved in Purkinje cell survival by suppressing the activity of caspase-3, a crucial mediator of apoptosis during cerebellar development. This review is an updated summary of the biosynthesis and biological actions of pineal neurosteroids.

Neuroestrogen, rapid action of estradiol, and GnRH neurons

Estradiol plays a pivotal role in the control of GnRH neuronal function, hence female reproduction. A series of recent studies in our laboratory indicate that rapid excitatory actions of estradiol directly modify GnRH neuronal activity in primate GnRH neurons through GPR30 and STX-sensitive receptors. Similar rapid direct actions of estradiol through estrogen receptor beta are also described in mouse GnRH neurons. In this review, we propose two novel hypotheses as a possible physiological role of estradiol in primates. First, while ovarian estradiol initiates the preovulatory GnRH surge through interneurons expressing estrogen receptor alpha, rapid direct membrane-initiated action of estradiol may play a role in sustaining GnRH surge release for many hours. Second, locally produced neuroestrogens may contribute to pulsatile GnRH release. Either way, estradiol synthesized in interneurons in the hypothalamus may play a significant role in the control of the GnRH surge and/or pulsatility of GnRH release.

Expression of P450c17 in the human fetal nervous system

P450c17 catalyzes steroid 17α-hydroxylase and 17,20 lyase activities. P450c17 is expressed in human fetal and postnatal adrenals and gonads and in the developing mouse nervous system, but little is known about its expression in the human nervous system. We obtained portions of 9-, 10-, and 11-wk gestation human fetuses and delineated the pattern of expression of P450c17 in their peripheral nervous systems by immunocytochemistry using the P450c17 antiserum previously used to characterize P450c17 in the mouse brain. P450c17 was readily detected in the dorsal root ganglia (DRG) and spinal cord. Neural structures were identified with antisera to the cytoskeletal protein neural cell adhesion molecule; DRG were identified with antisera to the neuronal transcription factor BRN3A and neurotrophin receptor tropomyosin-receptor-kinase B. The identification of P450c17 was confirmed using commercial antisera directed against different domains of P450c17 and by using antisera immunodepleted with authentic human P450c17. We also found expression of the P450 cholesterol side-chain cleavage enzyme (P450scc) in the spinal cord and DRG. Expression of P450scc is limited to cell bodies; unlike P450c17, we never detected P450scc in fiber tracts. Catalysis by P450c17 requires electron donation from P450 oxidoreductase (POR). Dual-label immunohistochemistry detected P450c17 and POR colocalized in DRG bundles, but some fibers containing P450c17 lacked POR. These data suggest that neurosteroids synthesized via these two enzymes may act in the developing human nervous system. The expression of P450c17 in structures lacking POR means that P450c17 may not be steroidogenic in those locations, suggesting that P450c17 may have additional functions that do not require POR.

Neurosteroidogenesis: insights from studies of songbirds

The long-held dogma that the brain is a target of steroids produced by peripheral organs has delayed the widespread acceptance of the functional importance of neurosteroidogenesis. Comparative studies have been vital for establishing the key actions of gonadal and adrenal hormones on brain and behaviour. No doubt, studies across diverse phyla will continue to be crucial for revealing the true significance of neurosteroidogenesis to proper function of the vertebrate brain. Here, we review work carried out in our laboratory, as well as in others, highlighting advances to our understanding of brain steroid synthesis and action using songbirds as animal models. These studies show that steroidogenic transporters and enzymes are present in the songbird brain and that their expression and/or activities are subject to developmental, seasonal or short-term regulation. Our work in a songbird points to synaptic synthesis of neuroactive steroids and fast, perisynaptic membrane actions. Combined with evidence for rapid steroidal control of behaviour, these studies firmly establish a neuromodulatory role for avian neurosteroids. We hope this work will join with that of other species to embolden the acceptance of neurosteroidal signalling as a core property of vertebrate neurobiology.

Sex-specific, rapid neuroestrogen fluctuations and neurophysiological actions in the songbird auditory forebrain

Recent evidence shows that brain-derived steroids such as estrogens ("neuroestrogens") are controlled in a manner very similar to traditional neurotransmitters. The advent of in vivo microdialysis for steroids in songbirds has provided new information about the spatial and temporal dynamics of neuroestrogen changes in a region of the auditory cortex, the caudomedial nidopallium (NCM). Here, experiments using in vivo microdialysis demonstrate that neuroestradiol (E(2)) fluctuations occur within the auditory NCM during presentation of naturalistic auditory and visual stimuli in males but only to the presentation of auditory stimuli in females. These changes are acute (within 30 min) and appear to be specific to the NCM, because similar treatments elicit no changes in E(2) in a nearby mesopallial region or in circulating plasma. Further experiments coupling in vivo steroid microdialysis with extracellular recordings in NCM show that neuroestrogens rapidly boost auditory responses to song stimuli in females, similar to recent observations in males. We also find that the rapid actions of estradiol on auditory responses are fully mimicked by the cell membrane-impermeable estrogen biotinylestradiol, consistent with acute estrogen actions at the neuronal membrane. Thus we conclude that local and acute E(2) flux is regulated by convergent multimodal sensory input, and that this regulation appears to be sex-specific. Second, rapid changes in local E(2) levels in NCM have consequences for the modulation of auditory processing in females and males. Finally, the rapid actions of neuroestrogens on NCM auditory processing appear to be mediated by a nonclassical, membrane-bound estrogen receptor.

Synaptocrine signaling: steroid synthesis and action at the synapse

Sex steroids have long been recognized for their dramatic impact on brain and behavior, including rapid modulation of membrane excitability. It is a widely held perception that these molecules are largely derived from peripheral sources and lack the spatial and temporal specificity ascribed to classical neuromodulatory systems. Neuromodulatory systems, in contrast, are defined by their regulated neuronal presynaptic secretion and by their functional modulation of perisynaptic events. Here we provide evidence for regulated presynaptic estrogen synthesis and functional postsynaptic actions. These results meet all the criteria for a neuromodulatory system and shift our perception of estrogens from that of peripheral signals exclusively to include that of a signaling system intrinsic to the brain itself. We apply the term synaptocrine to describe this form of neuromodulation.

DHEA and estradiol levels in brain, gonads, adrenal glands, and plasma of developing male and female European starlings

Traditionally, sexual differentiation of the brain was thought to be driven by gonadal hormones, particularly testosterone (T). However, recent studies in songbirds suggest that other steroids may also be important. For example, dehydroepiandrosterone (DHEA) can be synthesized by the gonads, adrenal glands, and/or brain and locally metabolized into T and 17β-estradiol (E(2)). Here, we examined DHEA and E(2) levels in the brain, peripheral tissues, and plasma of wild European starlings (Sturnus vulgaris). In Study 1, samples were collected from males and females at P0 (day of hatch), P6, and P8. In Study 2, samples were collected at P4. At P0, DHEA levels in the diencephalon were higher in males than females. DHEA levels were generally high in the gonads and adrenals, and they were higher in testes than ovaries at P8. Further, E(2) levels were non-detectable in most brain samples, suggesting that DHEA was not metabolized to E(2) or that locally produced E(2) was rapidly inactivated. At P4, DHEA levels in telencephalic regions were lower in males than females. Taken together, these data suggest that sex differences in peripheral DHEA secretion and neural DHEA metabolism at specific ages during development might play a role in sexual differentiation of the songbird brain.

Estradiol synthesis and action at the synapse: evidence for "synaptocrine" signaling

Classically, the modulation of brain function and behavior by steroid hormones was linked exclusively to secretion by peripheral endocrine glands. Subsequently, steroid actions within the brain were shown dependent upon either synthesis and secretion by peripheral organs or by production within the CNS itself using peripheral sources of precursors. Discovery of the estrogen-synthetic enzyme aromatase in brain further bolstered the latter view and served as a catalyst for expanding concepts of neurosteroidogenesis. In parallel research, several steroids, including estradiol, were found to have rapid effects on neuronal excitability, partially explained by novel actions at neuronal membranes. Recent findings from multiple levels of analysis and labs necessitate an updated view on how steroids are delivered to neural circuits. There is now considerable evidence for expression of the aromatase enzyme within synaptic boutons in the vertebrate CNS. Furthermore, additional work now directly couples rapid regulation of neuroestrogen synthesis with neurophysiological and behavioral outcomes. In this review we summarize evidence for targeted and acute synaptic estrogen synthesis and perisynaptic estrogen actions in the CNS of songbirds. We evaluate these findings in the context of criteria associated with classic neuromodulatory signaling. We term this novel form of signaling "synaptocrine," and discuss its implications.

Neurosteroid biosynthesis and function in the brain of domestic birds

It is now established that the brain and other nervous systems have the capability of forming steroids de novo, the so-called "neurosteroids." The pioneering discovery of Baulieu and his colleagues, using rodents, has opened the door to a new research field of "neurosteroids." In contrast to mammalian vertebrates, little has been known regarding de novo neurosteroidogenesis in the brain of birds. We therefore investigated neurosteroid formation and metabolism in the brain of quail, a domestic bird. Our studies over the past two decades demonstrated that the quail brain possesses cytochrome P450 side-chain cleavage enzyme (P450scc), 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4)-isomerase (3β-HSD), 5β-reductase, cytochrome P450 17α-hydroxylase/c17,20-lyase (P450(17α,lyase)), 17β-HSD, etc., and produces pregnenolone, progesterone, 5β-dihydroprogesterone (5β-DHP), 3β, 5β-tetrahydroprogesterone (3β, 5β-THP), androstenedione, testosterone, and estradiol from cholesterol. Independently, Schlinger's laboratory demonstrated that the brain of zebra finch, a songbird, also produces various neurosteroids. Thus, the formation and metabolism of neurosteroids from cholesterol is now known to occur in the brain of birds. In addition, we recently found that the quail brain expresses cytochrome P450(7α) and produces 7α- and 7β-hydroxypregnenolone, previously undescribed avian neurosteroids, from pregnenolone. This paper summarizes the advances made in our understanding of neurosteroid formation and metabolism in the brain of domestic birds. This paper also describes what are currently known about physiological changes in neurosteroid formation and biological functions of neurosteroids in the brain of domestic and other birds.

Genomic and neural analysis of the estradiol-synthetic pathway in the zebra finch

Background

Steroids are small molecule hormones derived from cholesterol. Steroids affect many tissues, including the brain. In the zebra finch, estrogenic steroids are particularly interesting because they masculinize the neural circuit that controls singing and their synthesis in the brain is modulated by experience. Here, we analyzed the zebra finch genome assembly to assess the content, conservation, and organization of genes that code for components of the estrogen-synthetic pathway and steroid nuclear receptors. Based on these analyses, we also investigated neural expression of a cholesterol transport protein gene in the context of song neurobiology.

Results

We present sequence-based analysis of twenty steroid-related genes using the genome assembly and other resources. Generally, zebra finch genes showed high homology to genes in other species. The diversity of steroidogenic enzymes and receptors may be lower in songbirds than in mammals; we were unable to identify all known mammalian isoforms of the 3β-hydroxysteroid dehydrogenase and 17β-hydroxysteroid dehydrogenase families in the zebra finch genome assembly, and not all splice sites described in mammals were identified in the corresponding zebra finch genes. We did identify two factors, Nobox and NR1H2-RXR, that may be important for coordinated transcription of multiple steroid-related genes. We found very little qualitative overlap in predicted transcription factor binding sites in the genes for two cholesterol transport proteins, the 18 kDa cholesterol transport protein (TSPO) and steroidogenic acute regulatory protein (StAR). We therefore performed in situ hybridization for TSPO and found that its mRNA was not always detected in brain regions where StAR and steroidogenic enzymes were previously shown to be expressed. Also, transcription of TSPO, but not StAR, may be regulated by the experience of hearing song.

Conclusions

The genes required for estradiol synthesis and action are represented in the zebra finch genome assembly, though the complement of steroidogenic genes may be smaller in birds than in mammals. Coordinated transcription of multiple steroidogenic genes is possible, but results were inconsistent with the hypothesis that StAR and TSPO mRNAs are co-regulated. Integration of genomic and neuroanatomical analyses will continue to provide insights into the evolution and function of steroidogenesis in the songbird brain.

Neural expression and post-transcriptional dosage compensation of the steroid metabolic enzyme 17beta-HSD type 4

Background

Steroids affect many tissues, including the brain. In the zebra finch, the estrogenic steroid estradiol (E₂) is especially effective at promoting growth of the neural circuit specialized for song. In this species, only the males sing and they have a much larger and more interconnected song circuit than females. Thus, it was surprising that the gene for 17β-hydroxysteroid dehydrogenase type 4 (HSD17B4), an enzyme that converts E₂ to a less potent estrogen, had been mapped to the Z sex chromosome. As a consequence, it was likely that HSD17B4 was differentially expressed in males (ZZ) and females (ZW) because dosage compensation of Z chromosome genes is incomplete in birds. If a higher abundance of HSD17B4 mRNA in males than females was translated into functional enzyme in the brain, then contrary to expectation, males could produce less E₂ in their brains than females.

Results

Here, we used molecular and biochemical techniques to confirm the HSD17B4 Z chromosome location in the zebra finch and to determine that HSD17B4 mRNA and activity were detectable in the early developing and adult brain. As expected, HSD17B4 mRNA expression levels were higher in males compared to females. This provides further evidence of the incomplete Z chromosome inactivation mechanisms in birds. We detected HSD17B4 mRNA in regions that suggested a role for this enzyme in the early organization and adult function of song nuclei. We did not, however, detect significant sex differences in HSD17B4 activity levels in the adult brain.

Conclusions

Our results demonstrate that the HSD17B4 gene is expressed and active in the zebra finch brain as an E₂ metabolizing enzyme, but that dosage compensation of this Z-linked gene may occur via post-transcriptional mechanisms.

Are rapid changes in gonadal testosterone release involved in the fast modulation of brain estrogen effects?

Estradiol facilitates the expression of male sexual behavior in Japanese quail within a few minutes. These rapid behavioral effects of estradiol could result from rapid changes in its local production in the preoptic area by aromatase, the enzyme converting testosterone into estradiol. Alternatively, aromatase activity may remain constant but fluctuations of local estradiol production could arise from rapid changes in the concentration of the enzymatic substrate, namely testosterone. Rapid increases of circulating testosterone levels have been observed in males of various species following social encounters. Surprisingly, in quail, the interaction with a female seems to result in a decrease in circulating testosterone levels. However, in that study conducted in quail, the samples were collected at intervals longer than the recently observed rapid effects of estradiol on sexual behavior. In the present study we investigated whether plasma testosterone concentrations fluctuate on a shorter time-frame. Eleven male were tested 5 min before and 5, 15 or 30 min after being allowed to have visual access to a female or to copulate with a female for 5 min. Both types of interactions resulted in a significant decline in circulating testosterone levels at latencies as short as 5 min. These data demonstrate that the decrease in testosterone levels is initiated shortly after sexual encounters. Because visual interactions with a female did not result in a rapid increase in testosterone concentrations, these findings rule out the possibility that a rapid rise in circulating testosterone levels participates in the rapid increase in brain estrogen synthesis and its facilitatory effects on copulatory behavior.

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.

Birdsong and the neural production of steroids

The forebrain circuits involved in singing and audition (the 'song system') in songbirds exhibit a remarkable capacity to synthesize and respond to steroid hormones. This review considers how local brain steroid production impacts the development, sexual differentiation, and activity of song system circuitry. The songbird forebrain contains all of the enzymes necessary for the de novo synthesis of steroids - including neuroestrogens - from cholesterol. Steroid production enzymes are found in neuronal cell bodies, but they are also expressed in pre-synaptic terminals in the song system, indicating a novel mode of brain steroid delivery to local circuits. The song system expresses nuclear hormone receptors, consistent with local action of brain-derived steroids. Local steroid production also occurs in brain regions that do not express nuclear hormone receptors, suggesting a non-classical mode of action. Recent evidence indicates that local steroid levels can change rapidly within the forebrain, in a manner similar to traditional neuromodulators. Lastly, we consider growing evidence for modulatory interactions between brain-derived steroids and neurotransmitter/neuropeptide networks within the song system. Songbirds have therefore emerged as a rich and powerful model system to explore the neural and neurochemical regulation of social behavior.

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.

Steroidogenic enzyme gene expression in the brain of the parthenogenetic whiptail lizard, Cnemidophorus uniparens

The steroidogenic enzyme CYP17 is responsible for catalyzing the production of androgenic precursors, while CYP19 converts testosterone to estradiol. De novo neurosteroidogenesis in specific brain regions influences steroid hormone dependent behaviors. In the all-female lizard species Cnemidophorus uniparens, individuals alternately display both male-like mounting and female-like receptivity. Mounting is associated with high circulating concentrations of progesterone following ovulation (PostOv), while receptivity is correlated with estrogen preceding it (PreOv). At a neuroanatomical level, the preoptic area (POA) and ventromedial nucleus of the hypothalamus (VMN) are the foci of the male-typical mounting and female-typical receptivity, respectively. In this study, we indirectly test the hypothesis that the whiptail lizard brain is capable of de novo neurosteroidogenesis by cloning fragments of the genes encoding two steroidogenic enzymes, CYP17 and CYP19, and examining their expression patterns in the C. uniparens brain. Our data indicate that these genes are expressed in the C. uniparens brain, and more importantly in the POA and VMN. Using radioactive in situ hybridization, we measured higher CYP17 mRNA levels in the POA of PostOv lizards compared to receptive PreOv animals; CYP19 mRNA levels in the VMN did not change across the ovarian cycle. To our knowledge, these are the first data suggesting that the reptilian brain is capable of de novo steroidogenesis. This study also supports the idea that non-gonadal sources of steroid hormones locally produced in behaviorally relevant brain loci are central to the mediation of behavioral output.

Forebrain steroid levels fluctuate rapidly during social interactions

Neurosteroids are powerful modulators of brain function and behavior, yet their dynamics within the brain have remained elusive. Using in vivo microdialysis in male zebra finches, we show that local estradiol levels increase rapidly within the forebrain during social interactions with females. Further, when males are exposed to other males’ song, local estradiol levels also increase and testosterone levels drop within a cortical/pallial auditory region that is analogous to mammalian auditory cortex. We also report that local estradiol and testosterone levels are differentially regulated in this same region by the conventional neurotransmitters glutamate and GABA, respectively. This study provides direct evidence that forebrain steroid levels are acutely and differentially regulated during social behavior, in a region-specific manner, and in a rapid time-course akin to that of traditional neuromodulators.

Combined effects of DHEA and fadrozole on aggression and neural VIP immunoreactivity in the non-breeding male song sparrow

The male Song Sparrow, Melospiza melodia morphna, shows high levels of aggression in its non-breeding season, concomitant with basal levels of circulating testosterone (T) and estradiol (E(2)). However, administration of fadrozole, an aromatase inhibitor, decreases non-breeding aggression in the field. Circulating levels of dehydroepiandrosterone (DHEA), an androgen/estrogen precursor, correspond to the seasonal expression of aggression in this species, with high levels in the breeding and non-breeding seasons when aggression is also high, and lower levels during the molt when aggression is low. We test the hypothesis that circulating DHEA up-regulates non-breeding aggression via an aromatase-mediated mechanism. We also hypothesize that this up-regulation of aggression is partially mediated by changes in vasoactive intestinal polypeptide (VIP) in the lateral extent of the bed nucleus of the stria terminalis (BSTl) and lateral septum (LS). Birds were administered either DHEA, fadrozole, or both for 2 weeks and tested for aggression in a lab-based paradigm. As predicted, birds given DHEA were significantly more aggressive. However, fadrozole did not block this effect, and, when administered without DHEA, also led to increased aggression over controls. These results may be explained by the fact that the behaviors measured in field tests, which include more direct attack behaviors, may be under different hormonal regulation than the behaviors measured in the lab paradigm, which represent warning, or threat, behaviors. VIP immunoreactivity (VIP-ir) changed across multiple brain regions with this treatment regimen, most notably in the LSO/VFI subdivision of the lateral septum.

Low sex steroids, high steroid receptors: Increasing the sensitivity of the nonreproductive brain

Male aggressive behavior is generally regulated by testosterone (T). In most temperate breeding males, aggressive behavior is only expressed during the reproductive period. At this time circulating T concentrations, brain steroid receptors, and steroid metabolic enzymes are elevated in many species relative to the nonreproductive period. Many tropical birds, however, display aggressive behavior both during the breeding and the nonbreeding season, but plasma levels of T can remain low throughout the year and show little seasonal fluctuation. Studies on the year-round territorial spotted antbird (Hylophylax n. naevioides) suggest that T nevertheless regulates aggressive behavior in both the breeding and nonbreeding season. We hypothesize that to regulate aggressive behaviors during the nonbreeding season, when T is at its minimum, male spotted antbirds increase brain sensitivity to steroids. This can be achieved by locally up-regulating androgen receptors (ARs), estrogen receptors (ERs), or the enzyme aromatase (AROM) that converts T into estradiol. We therefore compared mRNA expression of AR, ERalpha, and AROM in free- living male spotted antbirds across reproductive and nonreproductive seasons in two brain regions known to regulate both reproductive and aggressive behaviors. mRNA expression of ERalpha in the preoptic area and AR in the nucleus taeniae were elevated in male spotted antbirds during the nonbreeding season when circulating T concentrations were low. This unusual seasonal receptor regulation may represent a means for the year-round regulation of vertebrate aggressive behavior via steroids by increasing the brain's sensitivity to sex steroids during the nonbreeding season.

3beta-HSD activates DHEA in the songbird brain

Dehydroepiandrosterone (DHEA) is an abundant circulating prohormone in humans, with a variety of reported actions on central and peripheral tissues. Despite its abundance, the functions of DHEA are relatively unknown because common animal models (laboratory rats and mice) have very low DHEA levels in the blood. Over the past decade, we have obtained considerable evidence from avian studies demonstrating that (1) DHEA is an important circulating prohormone in songbirds and (2) the enzyme 3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD), responsible for converting DHEA into a more active androgen, is expressed at high levels in the songbird brain. Here, we first review biochemical and molecular studies demonstrating the widespread activity and expression of 3beta-HSD in the adult and developing songbird brain. Studies examining neural 3beta-HSD activity show effects of sex, stress, and season that are region-specific. Second, we review studies showing seasonal and stress-related changes in circulating DHEA in captive and wild songbird species. Third, we describe evidence that DHEA treatment can stimulate song behavior and the growth of neural circuits controlling song behavior. Importantly, brain 3beta-HSD and aromatase can work in concert to locally metabolize DHEA into active androgens and estrogens, which are critical for controlling behavior and robust adult neuroplasticity in songbirds. DHEA is likely secreted by the avian gonads and/or adrenals, as is the case in humans, but DHEA may also be synthesized de novo in the songbird brain from cholesterol or other precursors. Irrespective of its source, DHEA seems to be an important prohormone in songbirds, and 3beta-HSD is a key enzyme in the songbird brain.

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.

Activities of 3beta-HSD and aromatase in slices of developing and adult zebra finch brain

Sex steroids influence the development and function of the songbird brain. Developmentally, the neural circuitry underlying song undergoes masculine differentiation under the influence of estradiol. In adults, estradiol stimulates song behavior and the seasonal growth of song control circuits. There is good reason to believe that these neuroactive estrogens are synthesized in the brain. At all ages, estrogens could act at the lateral ventricle, during migration, or where song nuclei exist or will form. We investigated the activity of two critical steroidogenic enzymes, 3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD) and aromatase, using a slice culture system. Sagittal brain slices were collected from juvenile (posthatch day 20) and adult zebra finches containing either the lateral ventricle, where neurons are born, or the telencephalic song nuclei HVC and RA. The slices were incubated with (3)H-dehydroepiandrosterone or (3)H-androstenedione. Activity was determined by isolating certain products of 3beta-HSD (5alpha-androstanedione, 5beta-androstanedione, estrone, and estradiol) and aromatase (estrone and estradiol). Activities of both 3beta-HSD and aromatase were detected in all slices and were confirmed using specific enzyme inhibitors. We found no significant difference in activity between adult males and females in either region for either enzyme. Juvenile female slices containing the lateral ventricle, however, showed greater levels of 3beta-HSD activity than did similar slices from age-matched males. Determination of the activity of these critical steroidogenic enzymes in slice culture has implications for the role of neurosteroids in brain development.