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

Neuroestradiol and neuronal development: Not an exclusive male tale anymore

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

Evidence That Artificial Light at Night Induces Structure-Specific Changes in Brain Plasticity in a Diurnal Bird

We recently reported that artificial light at night (ALAN), at ecologically relevant intensities (1.5, 5 lux), increases cell proliferation in the ventricular zone and recruitment of new neurons in several forebrain regions of female zebra finches (Taeniopygia guttata), along with a decrease of total neuronal densities in some of these regions (indicating possible neuronal death). In the present study, we exposed male zebra finches to the same ALAN intensities, treated them with 5′-bromo-2′-deoxyuridine, quantified cell proliferation and neuronal recruitment in several forebrain regions, and compared them to controls that were kept under dark nights. ALAN increased cell proliferation in the ventricular zone, similar to our previous findings in females. We also found, for the first time, that ALAN increased new neuronal recruitment in HVC and Area X, which are part of the song system in the brain and are male-specific. In other brain regions, such as the medial striatum, nidopallium caudale, and hippocampus, we recorded an increased neuronal recruitment only in the medial striatum (unlike our previous findings in females), and relative to the controls this increase was less prominent than in females. Moreover, the effect of ALAN duration on total neuronal densities in the studied regions varied between the sexes, supporting the suggestion that males are more resilient to ALAN than females. Suppression of nocturnal melatonin levels after ALAN exhibited a light intensity-dependent decrease in males in contrast to females, another indication that males might be less affected by ALAN. Taken together, our study emphasizes the importance of studying both sexes when considering ALAN effects on brain plasticity.

Questioning Seasonality of Neuronal Plasticity in the Adult Avian Brain

To date, studies that reported seasonal patterns of adult neurogenesis and neuronal recruitment have correlated them to seasonal behaviors as the cause or as a consequence of neuronal changes. The aim of our study was to test this correlation, and to investigate whether there is a seasonal pattern of new neuronal recruitment that is not correlated to behavior. To do this, we used adult female zebra finches (songbirds that are not seasonal breeders), kept them under constant social, behavioral, and spatial environments, and compared neuronal recruitment in their brains during two seasons, under natural and laboratory conditions. Under natural conditions, no significant differences were found in the pattern of new neuronal recruitment across seasons. However, under artificial indoor conditions that imitated the natural conditions, higher neuronal recruitment occurred in late summer (August) compared to early spring (February). Moreover, our data indicate that "mixing" temperature and day length significantly reduces new neuronal recruitment, demonstrating the importance of the natural combination of temperature and day length. Taken together, our findings show, for the first time, that neuroplasticity changes under natural vs. artificial conditions, and demonstrate the importance of both laboratory and field experiments when looking at complex biological systems.

Sex differences in hippocampal mineralocorticoid and glucocorticoid receptor mRNA expression in response to acute mate pair separation in zebra finches (Taeniopygia guttata)

Mate separation has been shown to mediate changes in physiological and behavioral processes via activation of the hypothalamo-pituitary-adrenal (HPA) axis in both mammalian and avian species. To elucidate the neural mechanisms associated with changes in the HPA axis in response to social stress, we investigated the effects of mate pair separation on circulating corticosterone concentrations as well as gene expression levels of mineralocorticoid receptor (MR), glucocorticoid receptor (GR), and corticotropin releasing hormone (CRH) in the hypothalamus and hippocampus of both male and female zebra finches, a species that forms strong pair bonds. Zebra finches (Taeniopygia guttata) were housed three to a cage (a mated pair plus a stimulus female), and were assigned to one of three new housing treatment groups: (1) male or female removed from their respective mate and placed in a cage with a new opposite sex conspecific and stimulus female (2) male or female that remained with their mate, but a new stimulus female was introduced, or (3) the subjects were handled but not separated from their mate or the stimulus female. After 48 hr in the new housing condition, we observed significant increases in plasma corticosterone concentrations in response to both mate pair and stimulus female separation. No significant differences in MR, GR, or CRH mRNA expression in the hypothalamus were observed in response to any treatment for both males and females. Females exhibited a significant up regulation in hippocampal MR, but not GR mRNA, whereas males exhibited a significant down regulation of both hippocampal MR and GR mRNA in response to mate pair separation. Thus, the hippocampus appears to play a key role in regulating sex specific responses to social stressors.

11β-HSD Types 1 and 2 in the Songbird Brain

Glucocorticoid (GC) hormones act on the brain to regulate diverse functions, from behavior and homeostasis to the activity of the hypothalamic-pituitary-adrenal axis. Local regeneration and metabolism of GCs can occur in target tissues through the actions of the 11β-hydroxysteroid dehydrogenases [11 beta-hydroxysteroid dehydrogenase type 1 (11β-HSD1) and 11 beta-hydroxysteroid dehydrogenase type 2 (11β-HSD2), respectively] to regulate access to GC receptors. Songbirds have become especially important model organisms for studies of stress hormone action; however, there has been little focus on neural GC metabolism. Therefore, we tested the hypothesis that 11β-HSD1 and 11β-HSD2 are expressed in GC-sensitive regions of the songbird brain. Localization of 11β-HSD expression in these regions could provide precise temporal and spatial control over GC actions. We quantified GC sensitivity in zebra finch (Taeniopygia guttata) brain by measuring glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) expression across six regions, followed by quantification of 11β-HSD1 and 11β-HSD2 expression. We detected GR, MR, and 11β-HSD2 mRNA expression throughout the adult brain. Whereas 11β-HSD1 expression was undetectable in the adult brain, we detected low levels of expression in the brain of developing finches. Across several adult brain regions, expression of 11β-HSD2 covaried with GR and MR, with the exception of the cerebellum and hippocampus. It is possible that receptors in these latter two regions require direct access to systemic GC levels. Overall, these results suggest that 11β-HSD2 expression protects the adult songbird brain by rapid metabolism of GCs in a context and region-specific manner.

Cellular and molecular attributes of neural stem cell niches in adult zebrafish brain

Adult neurogenesis is a complex, presumably conserved phenomenon in vertebrates with a broad range of variations regarding neural progenitor/stem cell niches, cellular composition of these niches, migratory patterns of progenitors and so forth among different species. Current understanding of the reasons underlying the inter-species differences in adult neurogenic potential, the identification and characterization of various neural progenitors, characterization of the permissive environment of neural stem cell niches and other important aspects of adult neurogenesis is insufficient. In the last decade, zebrafish has emerged as a very useful model for addressing these questions. In this review, we have discussed the present knowledge regarding the neural stem cell niches in adult zebrafish brain as well as their cellular and molecular attributes. We have also highlighted their similarities and differences with other vertebrate species. In the end, we shed light on some of the known intrinsic and extrinsic factors that are assumed to regulate the neurogenic process in adult zebrafish brain. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1188-1205, 2017.

Studies of HVC Plasticity in Adult Canaries Reveal Social Effects and Sex Differences as Well as Limitations of Multiple Markers Available to Assess Adult Neurogenesis

In songbirds, neurogenesis in the song control nucleus HVC is sensitive to the hormonal and social environment but the dynamics of this process is difficult to assess with a single exogenous marker of new neurons. We simultaneously used three independent markers to investigate HVC neurogenesis in male and female canaries. Males were castrated, implanted with testosterone and housed either alone (M), with a female (M-F) or with another male (M-M) while females were implanted with 17β-estradiol and housed with a male (F-M). All subjects received injections of the two thymidine analogues, BrdU and of EdU, respectively 21 and 10 days before brain collection. Cells containing BrdU or EdU or expressing doublecortin (DCX), which labels newborn neurons, were quantified. Social context and sex differentially affected total BrdU+, EdU+, BrdU+EdU- and DCX+ populations. M-M males had a higher density of BrdU+ cells in the ventricular zone adjacent to HVC and of EdU+ in HVC than M-F males. M birds had a higher ratio of BrdU+EdU- to EdU+ cells than M-F subjects suggesting higher survival of newer neurons in the former group. Total number of HVC DCX+ cells was lower in M-F than in M-M males. Sex differences were also dependent of the type of marker used. Several technical limitations associated with the use of these multiple markers were also identified. These results indicate that proliferation, recruitment and survival of new neurons can be independently affected by environmental conditions and effects can only be fully discerned through the use of multiple neurogenesis markers.

Dehydroepiandrosterone Heightens Aggression and Increases Androgen Receptor and Aromatase mRNA Expression in the Brain of a Male Songbird

Dehydroepiandrosterone (DHEA) is a testosterone/oestrogen precursor and known modulator of vertebrate aggression. Male song sparrows (Melospiza melodia morphna) show high aggression during breeding and nonbreeding life-history stages when circulating DHEA levels are high, and low aggression during molt when DHEA levels are low. We previously showed that androgen receptor and aromatase mRNA expression are higher during breeding and/or nonbreeding in brain regions associated with reproductive and aggressive behaviour, although the potential role of DHEA in mediating these seasonal changes remained unclear. In the present study, nonbreeding male song sparrows were captured and held in the laboratory under short days (8 : 16 h light/dark cycle) and implanted with s.c. DHEA-filled or empty (control) implants for 14 days. DHEA implants increased aggression in a laboratory-based simulated territorial intrusion. Brains of DHEA-implanted birds showed higher aromatase mRNA expression in the preoptic area (POA) and higher androgen receptor mRNA expression in the periventricular nucleus of the medial striatum (pvMSt) and ventromedial nucleus of the hypothalamus. The DHEA-induced increases in aromatase expression in the POA and androgen receptor expression in the pvMSt are consistent with previously reported seasonal increases in these markers associated with naturally elevated DHEA levels. This suggests that DHEA facilitates seasonal increases in aggression in nonbreeding male song sparrows by up-regulating steroid signalling/synthesis machinery in a brain region-specific fashion.

Determinants and significance of corticosterone regulation in the songbird brain

Songbirds exhibit significant adult neuroplasticity that, together with other neural specializations, makes them an important model system for neurobiological studies. A large body of work also points to the songbird brain as a significant target of steroid hormones, including corticosterone (CORT), the primary avian glucocorticoid. Whereas CORT positively signals the brain for many functions, excess CORT may interfere with natural neuroplasticity. Consequently, mechanisms may exist to locally regulate CORT levels in brain to ensure optimal concentrations. However, most studies in songbirds measure plasma CORT as a proxy for levels at target tissues. In this paper, we review literature concerning circulating CORT and its effects on behavior in songbirds, and discuss recent work suggesting that brain CORT levels are regulated independently of changes in adrenal secretion. We review possible mechanisms for CORT regulation in the avian brain, including corticosteroid-binding globulins, p-glycoprotein activity in the blood-brain barrier and CORT metabolism by the 11ß hydroxysteroid dehydrogenases. Data supporting a role for CORT regulation within the songbird brain have only recently begun to emerge, suggesting that this is an avenue for important future research.

Effects of corticosterone and DHEA on doublecortin immunoreactivity in the song control system and hippocampus of adult song sparrows

Adult neuroplasticity is strongly influenced by steroids. In particular, corticosterone (CORT) and dehydroepiandrosterone (DHEA) can have opposing effects, where CORT reduces while DHEA increases neurogenesis and neuron recruitment. It has been previously shown that in adult male song sparrows, DHEA treatment increases neuron recruitment throughout the telencephalon, including the lateral ventricular zone, while the effect of CORT treatment is restricted to HVC, one of the song control regions. These data suggest that the two steroids may differentially affect proliferation, migration, differentiation, and/or survival of new neurons. To determine if CORT or DHEA alters the migration and differentiation of young neurons, we examined an endogenous marker of migrating immature neurons, doublecortin (DCX), in HVC and hippocampus of adult male song sparrows that were treated with CORT and/or DHEA for 28 days. In HVC, DHEA increased the number of DCX-labeled round cells, while CORT had no main effect on the number of DCX-labeled cells. Furthermore, DHEA increased the area covered by DCX immunoreactivity in HVC, regardless of CORT treatment. In the hippocampus, neither DHEA nor CORT affected DCX immunoreactivity. These results suggest that DHEA enhances migration and differentiation of young neurons into HVC while CORT does not affect the process, whether in the presence of DHEA or not.

Neurogenesis in the adult avian song-control system

New neurons are added throughout the forebrain of adult birds. The song-control system is a model to investigate the addition of new long-projection neurons to a cortical circuit that regulates song, a learned sensorimotor behavior. Neuroblasts destined for the song nucleus HVC arise in the walls of the lateral ventricle, and wander through the pallium to reach HVC. The survival of new HVC neurons is supported by gonadally secreted testosterone and its downstream effectors including neurotrophins, vascularization, and electrical activity of postsynaptic neurons in nucleus RA (robust nucleus of the arcopallium). In seasonal species, the HVC→RA circuit degenerates in nonbreeding birds, and is reconstructed by the incorporation of new projection neurons in breeding birds. There is a functional linkage between the death of mature HVC neurons and the birth of new neurons. Various hypotheses for the function of adult neurogenesis in the song system can be proposed, but this remains an open question.

Anatomically discrete sex differences and enhancement by testosterone of cell proliferation in the telencephalic ventricle zone of the adult canary brain

Previous work in songbirds has suggested that testosterone increases neuronal recruitment and survival in HVC but does not affect neuronal proliferation in the ventricular zone and that males and females have similar rates of proliferation except at discrete locations. Many of these conclusions are however based on limited data or were inferred indirectly. Here we specifically tested the effects of testosterone on cellular proliferation in the ventricular zone of both male and female adult canaries. We implanted adult birds of both sexes with testosterone or empty implants for 1 week and injected them with BrdU. One day later, we collected their brains and quantified BrdU-positive cells in the ventricular zone (VZ) at different rostro-caudal levels of the brain, ranging from the level where the song nucleus Area X occurs through the caudal extent of HVC. Proliferation in the dorsal part of the VZ was low and unaffected by sex or testosterone treatment. In the ventral part of the VZ, females had more proliferating cells than males, but only at rostral levels, near Area X. Also in the ventral part of the VZ, testosterone increased proliferation in birds of both sexes, but only in the mid- to caudal-VZ, caudal to the level of Area X, around the septum and HVC. We thus demonstrate here that there is both an effect of testosterone and possibly a more subtle effect of sex on cellular proliferation in the adult songbird brain, and that these effects are specific to different levels of the brain.

Seasonal changes in patterns of gene expression in avian song control brain regions

Photoperiod and hormonal cues drive dramatic seasonal changes in structure and function of the avian song control system. Little is known, however, about the patterns of gene expression associated with seasonal changes. Here we address this issue by altering the hormonal and photoperiodic conditions in seasonally-breeding Gambel's white-crowned sparrows and extracting RNA from the telencephalic song control nuclei HVC and RA across multiple time points that capture different stages of growth and regression. We chose HVC and RA because while both nuclei change in volume across seasons, the cellular mechanisms underlying these changes differ. We thus hypothesized that different genes would be expressed between HVC and RA. We tested this by using the extracted RNA to perform a cDNA microarray hybridization developed by the SoNG initiative. We then validated these results using qRT-PCR. We found that 363 genes varied by more than 1.5 fold (>log₂ 0.585) in expression in HVC and/or RA. Supporting our hypothesis, only 59 of these 363 genes were found to vary in both nuclei, while 132 gene expression changes were HVC specific and 172 were RA specific. We then assigned many of these genes to functional categories relevant to the different mechanisms underlying seasonal change in HVC and RA, including neurogenesis, apoptosis, cell growth, dendrite arborization and axonal growth, angiogenesis, endocrinology, growth factors, and electrophysiology. This revealed categorical differences in the kinds of genes regulated in HVC and RA. These results show that different molecular programs underlie seasonal changes in HVC and RA, and that gene expression is time specific across different reproductive conditions. Our results provide insights into the complex molecular pathways that underlie adult neural plasticity.

Expression patterns of mineralocorticoid and glucocorticoid receptors in Bengalese finch (Lonchura striata var. domestica) brain suggest a relationship between stress hormones and song-system development

Much evidence suggests that song traits function as an honest signal of male quality during mate choice in songbirds. Because songbirds learn vocalizations during the juvenile stage, development of the song system and song traits is affected by stressful conditions. However, it remains unknown how stressful conditions affect later song traits during development. To explore the relationship between glucocorticoids and song-system development, we performed in situ hybridization analysis of the glucocorticoid and mineralocorticoid receptors in juvenile and adult brains. The glucocorticoid receptor showed weak expression in song nuclei and strong expression in the hypothalamus, whereas the mineralocorticoid receptor showed strong song-nuclei-related expression. Thus, it appears that glucocorticoids are involved in song development directly by binding to receptors in song nuclei or indirectly by regulating sex hormones through hypothalamic hormones.

Glucocorticoid receptor blockade inhibits brain cell addition and aggressive signaling in electric fish, Apteronotus leptorhynchus

When animals are under stress, glucocorticoids commonly inhibit adult neurogenesis by acting through glucocorticoid receptors (GRs). However, in some cases, conditions that elevate glucocorticoids promote adult neurogenesis, and the role of glucocorticoid receptors in these circumstances is not well understood. We examined the involvement of GRs in social enhancement of brain cell addition and aggressive signaling in electric fish, Apteronotus leptorhynchus. In this species, long-term social interaction simultaneously elevates plasma cortisol, enhances brain cell addition and increases production of aggressive electrocommunication signals ("chirps"). We implanted isolated and paired fish with capsules containing nothing (controls) or the GR antagonist, RU486, recorded chirp production and locomotion for 7d, and measured the density of newborn cells in the periventricular zone. Compared to isolated controls, paired controls showed elevated chirping in two phases: much higher chirp rates in the first 5h and moderately higher nocturnal rates thereafter. Treating paired fish with RU486 reduced chirp rates in both phases to those of isolated fish, demonstrating that GR activation is crucial for socially induced chirping. Neither RU486 nor social interaction affected locomotion. RU486 treatment to paired fish had a partial effect on cell addition: paired RU486 fish had less cell addition than paired control fish but more than isolated fish. This suggests that cortisol activation of GRs contributes to social enhancement of cell addition but works in parallel with another GR-independent mechanism. RU486 also reduced cell addition in isolated fish, indicating that GRs participate in the regulation of cell addition even when cortisol levels are low.

Localization and sex-difference of steroid receptor coactivator-1 immunoreactivities in the brain of adult female and male mice

Females and males are different in brain and behaviors. These differences are mediated by steroids and their nuclear receptors which require coactivators to regulate the transcription of target genes. Studies have shown that these coactivators are critical for modulating steroid hormone action in the brain. Steroid receptor coactivator-1 has been implied in the regulation of reproduction, stress, motor learning, and limited studies have reported the sex-specific difference of SRC-1 mRNA or protein expression in specific brain regions, but the expression and differences of SRC-1 immunoreactivities in adult female and male brain remain unclear. In this study we reported that in both sexes, high levels of SRC-1 immunoreactivities were detected in olfactory bulb, cerebral cortex, hippocampus, Purkinje cells, some limited diencephalon and brainstem nuclei. The immunopositive materials were predominantly detected in cell nucleus, but in some regions they were also detected in the processes or fiber-like structures. In most of the brain regions studied, males possessed significantly higher levels of SRC-1 immunoreactivities than that of females. Higher levels of SRC-1 were detected in some nuclei related to learning and memory, motor regulation and reproduction indicated its potential roles in neurodegeneration and sex-dependent behavior and structure; the region- and sex-specific localization of SRC-1 immunoreactivities in agreement with that of some steroid receptors, indicating this coactivator play important roles in these hormone-reactive regions and cell groups related to reproduction, learning and memory, integration of motor and sense.

Corticosterone and dehydroepiandrosterone have opposing effects on adult neuroplasticity in the avian song control system

Chronic elevations in glucocorticoids can decrease the production and survival of new cells in the adult brain. In rat hippocampus, supraphysiological doses of dehydroepiandrosterone (DHEA; a sex steroid precursor synthesized in the gonads, adrenals, and brain) have antiglucocorticoid properties. With male song sparrows (Melospiza melodia), we examined the effects of physiological doses of corticosterone, the primary circulating glucocorticoid in birds, and DHEA on adult neuroplasticity. We treated four groups of nonbreeding sparrows for 28 days with empty (control), corticosterone, DHEA, or corticosterone + DHEA implants. Subjects were injected with BrdU on days 3 and 4. In HVC, a critical song control nucleus, corticosterone and DHEA had independent, additive effects. Corticosterone decreased, whereas DHEA increased, HVC volume, NeuN(+) cell number, and BrdU(+) cell number. Coadministration of DHEA completely reversed the neurodegenerative effects of chronic corticosterone treatment. In an efferent target of HVC, the robust nucleus of the arcopallium (RA), DHEA increased RA volume, but this effect was blocked by coadministration of corticosterone. There were similar antagonistic interactions between corticosterone and DHEA on BrdU(+) cell number in the hippocampus and ventricular zone. This is the first report on the effects of corticosterone treatment on the adult song control circuit, and HVC was the most corticosterone-sensitive song nucleus examined. In HVC, DHEA is neuroprotective and counteracts several pronounced effects of corticosterone. Within brain regions that are particularly vulnerable to corticosterone, such as the songbird HVC and rat hippocampus, DHEA appears to be a potent native antiglucocorticoid.

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

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

Opioid modulation of cell proliferation in the ventricular zone of adult zebra finches (Taenopygia guttata)

Besides modulating pain, stress, physiological functions, motivation, and reward, the opioid system has been implicated in developmental and adult mammalian neurogenesis and gliogenesis. In adult male songbirds including zebra finches, neurons generated from the ventricular zone (VZ) of the lateral ventricles are incorporated throughout the telencephalon, including the song control nuclei, HVC, and area X. Although the endogenous opioid met-enkephalin is present in neurons adjacent to the VZ and is upregulated in song control regions during singing, it is not known whether the opioid system can modulate adult neurogenesis/gliogenesis in zebra finches. We used quantitative RT-PCR and in situ hybridization to demonstrate that μ- and δ-opioid receptors are expressed by the VZ of adult male zebra finches. Treating cultured VZ cells from male birds with the opioid antagonist naloxone led to an increase in cell proliferation measured by 5-bromo-2-deoxyuridine incorporation, whereas administering met-enkephalin had the opposite effect, compared with saline-treated cultures. Systemically administering naloxone (2.5 mg/kg body wt) to adult male zebra finches for 4 d also led to a significant increase in cell proliferation in the ventral VZ of these birds, compared with saline-treated controls. Our results show that cell proliferation is augmented by naloxone in the VZ adjacent to the anterior commissure, suggesting that the endogenous opioids modulate adult neurogenesis/gliogenesis by inhibiting cell proliferation in songbirds.

11beta-hydroxysteroid dehydrogenase type 2 in zebra finch brain and peripheral tissues

The enzyme 11betaHSD2 inactivates glucocorticoids by synthesizing metabolites that bind poorly to mineralocorticoid and glucocorticoid receptors. Oscine songbirds (Passeriformes) are important models for investigating stress hormone effects on brain and behavior but nothing is known about 11betaHSD2 activity in the songbird brain. We measured 11betaHSD2 mRNA expression and enzymatic activity in brain of adult and developing male and female zebra finches. Since 11betaHSD2 plays an important role in GC metabolism in some peripheral organs we measured mRNA and catalytic activity also in the adult liver, kidney colon and gonads. 11betaHSD2 mRNA was detected in all brain regions examined with expression in the cerebellum and hypothalamus greater in females than in males; expression in ovaries was greater than in testes. No differences were detected in the other peripheral tissues. Catalytic activity of 11betaHSD2 could be measured in brain, but at low levels and no sex differences were measured in any region tested. Because 11betaHSD2 protects mineralocorticoid sensitive tissues from inappropriate CORT action, we also measured mineralocorticoid receptor (MR) expression in adult brain kidney and liver. MR mRNA was detected in all tissues with similar levels of expression in neural and peripheral tissues. The wide distribution of 11betaHSD2 and MR throughout the songbird brain suggests that concentrations of glucocorticoids may be locally regulated in brain to modulate their actions on MR and possibly also glucocorticoid receptors (GR). Notable differences between mRNA expression and activity point to post-transcriptional regulation of the 11betaHSD2 enzyme.

Sex-specific therapeutic strategies based on neuroactive steroids: In search for innovative tools for neuroprotection

Different pathologies of the central and peripheral nervous system show sex differences in their incidence, symptomatology and/or neurodegenerative outcome. These include Parkinson's disease, Alzheimer's disease, Huntington's disease, multiple sclerosis, traumatic brain injury, stroke, autism, schizophrenia, depression, anxiety disorders, eating disorders and peripheral neuropathy. These sex differences reveal the need for sex-specific neuroprotective strategies. This review article and other manuscripts published in this issue of Hormones and Behavior analyze possible sex-specific therapeutic strategies based on neuroactive steroids. In particular in our introductory article, the possibility that sex differences in the levels or in the action of neuroactive steroids may represent causative factors for sex differences in the incidence or manifestation of pathologies of the nervous system is considered.

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.