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

Estrogen Formation and Inactivation Following TBI: What we Know and Where we Could go

Traumatic brain injury (TBI) is responsible for various neuronal and cognitive deficits as well as psychosocial dysfunction. Characterized by damage inducing neuroinflammation, this response can cause an acute secondary injury that leads to widespread neurodegeneration and loss of neurological function. Estrogens decrease injury induced neuroinflammation and increase cell survival and neuroprotection and thus are a potential target for use following TBI. While much is known about the role of estrogens as a neuroprotective agent following TBI, less is known regarding their formation and inactivation following damage to the brain. Specifically, very little is known surrounding the majority of enzymes responsible for the production of estrogens. These estrogen metabolizing enzymes (EME) include aromatase, steroid sulfatase (STS), estrogen sulfotransferase (EST/SULT1E1), and some forms of 17β-hydroxysteroid dehydrogenase (HSD17B) and are involved in both the initial conversion and interconversion of estrogens from precursors. This article will review and offer new prospective and ideas on the expression of EMEs following TBI.

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.

Birdsong as a window into language origins and evolutionary neuroscience

Humans and songbirds share the key trait of vocal learning, manifested in speech and song, respectively. Striking analogies between these behaviours include that both are acquired during developmental critical periods when the brain's ability for vocal learning peaks. Both behaviours show similarities in the overall architecture of their underlying brain areas, characterized by cortico-striato-thalamic loops and direct projections from cortical neurons onto brainstem motor neurons that control the vocal organs. These neural analogies extend to the molecular level, with certain song control regions sharing convergent transcriptional profiles with speech-related regions in the human brain. This evolutionary convergence offers an unprecedented opportunity to decipher the shared neurogenetic underpinnings of vocal learning. A key strength of the songbird model is that it allows for the delineation of activity-dependent transcriptional changes in the brain that are driven by learned vocal behaviour. To capitalize on this advantage, we used previously published datasets from our laboratory that correlate gene co-expression networks to features of learned vocalization within and after critical period closure to probe the functional relevance of genes implicated in language. We interrogate specific genes and cellular processes through converging lines of evidence: human-specific evolutionary changes, intelligence-related phenotypes and relevance to vocal learning gene co-expression in songbirds. This article is part of the theme issue 'What can animal communication teach us about human language?'

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.

Dynamic variation in forebrain estradiol levels during song learning

Estrogens shape brain circuits during development, and the capacity to synthesize estrogens locally has consequences for both sexual differentiation and the acute modulation of circuits during early learning. A recently optimized method to detect and quantify fluctuations in brain estrogens in vivo provides a direct means to explore how brain estrogen production contributes to both differentiation and neuromodulation during development. Here, we use this method to test the hypothesis that neuroestrogens are sexually differentiated as well as dynamically responsive to song tutoring (via passive video/audio playback) during the period of song learning in juvenile zebra finches. Our results show that baseline neuroestradiol levels in the caudal forebrain do not differ between males and females during an early critical masculinization window. Instead, we observe a prominent difference between males and females in baseline neuroestradiol that emerges during the subadult stage as animals approach sexual maturity. Second, we observe that fluctuating neuroestradiol levels during periods of passive song tutoring exhibit a markedly different profile in juveniles as compared to adults. Specifically, neuroestrogens in the caudal forebrain are elevated following (rather than during) tutor song exposure in both juvenile males and females, suggesting an important role for the early consolidation of tutor song memories. These results further reveal a circadian influence on the fluctuations in local neuroestrogens during sensory/cognitive tasks. Taken together, these findings uncover several unexpected features of brain estrogen synthesis in juvenile animals that may have implications for secondary masculinization as well as the consolidation of recent sensory experiences.

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.

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.

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.

17β-Hydroxysteroid dehydrogenase type IV, a Z-linked gene, is higher in females than in males in visual and auditory regions of developing zebra finches

One of the most important decisions in a monogamous animal's life is the choice of a partner (partner preference), but the process by which this occurs remains poorly understood. The present study tests the hypothesis that hormones and genes play a role in sexual differentiation of partner preferences, as in the song system. We focused on a Z-linked gene, 17β-hydroxysteroid dehydrogenase type IV (HSD17B4), coding for a steroidogenic enzyme that converts estradiol (E2) into an inactive metabolite. HSD17B4 mRNA is expressed more in the song regions of males compared to females throughout development, suggesting that regulation of E2 is important for male-typical song development. Here, we focused on four regions associated with sexual partner preferences. Females had significantly higher levels of HSD17B4 mRNA in auditory (caudomedial nidopallium) and visual (hyperpallium apicale) regions than did males at day 25. HSD17B4 was expressed in the hippocampus and caudolateral nidopallium, but there were no sex differences. In a second experiment, animals of both sexes were treated with E2 and HSD17B4 and androgen receptor (AR) mRNA were measured, since masculinization of the song system is, in part, accomplished by AR. AR was low across the four regions and was not sexually differentiated. E2 treatments increased HSD17B4 mRNA in the auditory region of males, which is contrary to findings in the song system. Our research suggests that different behaviors may be guided by the same genes and hormones, but that the exact nature of the gene-hormone relationships may differ according to brain region and behavior.

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.

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.

The effects of estradiol on 17β-hydroxysteroid dehydrogenase type IV and androgen receptor expression in the developing zebra finch song system

Recent work in zebra finches suggests that genes and hormones may act together to masculinize the brain. This study tested the effects of exogenous estradiol (E2) on 17β-hydroxysteroid dehydrogenase type IV (HSD17B4) and the co-localization of HSD17B4 and androgen receptor (AR) mRNA. We asked three primary questions: First, how does post-hatching E2 treatment affect HSD17B4 mRNA expression in males and females? Second, is this gene expressed in the same cells as AR. Third, if so does E2 modulate co-expression? Female finches implanted with 50 μg of E2 on the third day post-hatching showed a significant increase in the density of cells expressing HSD17B4 and AR in HVC at day 25. Co-localization of AR cells that also expressed HSD17B4 was high across groups (>81%). We found significant sex differences in co-localization in both the HVC and Area X of control animals, with males showing a higher percentage of cells expressing AR mRNA that also expressed HSD17B4 in comparison to females. However, although E2 treatments significantly increased the number of cells expressing HSD17B4 mRNA and AR mRNA in the HVC of females, the percentage of HSD17B4 cells co-expressing AR was reduced in HVC and Area X in E2-treated animals. These results lend support to the hypothesis that genes and hormones may act in concert to modulate the sexually differentiation of the zebra finch song system. Further, the data suggest that a single hormonal mechanism cannot mimic the complex development of male singing behavior and associated song nuclei.

Neuroendocrine contributions to sexual partner preference in birds

A majority of birds are socially monogamous, providing exceptional opportunities to discover neuroendocrine mechanisms underlying preferences for opposite-sex partners where the sexes form extended affiliative relationships. Zebra finches have been the focus of the most systematic program of research to date in any socially monogamous animal. In this species, sexual partner preference can be partially or largely sex reversed with hormone manipulations during early development, suggesting a role for organizational hormone actions. This same conclusion emerges from research with Japanese quail, which do not form long-term pairs. In zebra finches, social experience manipulations during juvenile development also can sex reverse partner preference, either alone or in combination with an early hormone manipulation. Although there are several candidate brain regions where neural mechanisms could underlie these effects of hormones or social experience, the necessary research has not yet been done to determine their involvement. The neuroendocrinology of avian sexual partner preference is still frontier territory.

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.

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.