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

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.

Improved zebra finch brain transcriptome identifies novel proteins with sex differences

The zebra finch (Taeniopygia guttata), a representative oscine songbird species, has been widely studied to investigate behavioral neuroscience, most notably the neurobiological basis of vocal learning, a rare trait shared in only a few animal groups including humans. In 2019, an updated zebra finch genome annotation (bTaeGut1_v1.p) was released from the Ensembl database and is substantially more comprehensive than the first version published in 2010. In this study, we utilized the publicly available RNA-seq data generated from Illumina-based short-reads and PacBio single-molecule real-time (SMRT) long-reads to assess the bird transcriptome. To analyze the high-throughput RNA-seq data, we adopted a hybrid bioinformatic approach combining short and long-read pipelines. From our analysis, we added 220 novel genes and 8,134 transcript variants to the Ensembl annotation, and predicted a new proteome based on the refined annotation. We further validated 18 different novel proteins by using mass-spectrometry data generated from zebra finch caudal telencephalon tissue. Our results provide additional resources for future studies of zebra finches utilizing this improved bird genome annotation and proteome.

A transient reduction in circulating corticosterone reduces object neophobia in male house sparrows

Aversive reactions to novelty (or "neophobia") have been described in a wide variety of different animal species and can affect an individual's ability to exploit new resources and avoid potential dangers. However, despite its ecological importance, the proximate causes of neophobia are poorly understood. In this study, we tested the role of glucocorticoid hormones in neophobia in wild-caught house sparrows (Passer domesticus, n = 11 males) by giving an injection of the drug mitotane that reduced endogenous corticosterone for several days or a vehicle control, and then examined the latency to feed when the food dish was presented with or without a novel object in, on, or near the dish. Each sparrow was exposed to multiple novel object and control trials and received both vehicle control and mitotane treatments, with a week between treatments to allow the drug to wash out. As found previously, all novel objects significantly increased sparrows' latency to feed compared to no object present. Reducing corticosterone using mitotane significantly reduced the latency to feed in the presence of novel objects. In control trials without objects, mitotane had no significant effects on feeding time. Although we have shown that corticosterone affects neophobia, further studies using specific receptor agonists and antagonists will help clarify the neurobiological mechanisms involved and determine whether baseline or stress-induced corticosterone is driving this effect. These results suggest that increased glucocorticoids (e.g., due to human-induced stressors) could increase neophobia, affecting the ability of individuals to exploit novel resources, and, ultimately, to persist in human-altered environments.

Hypothalamic-pituitary-adrenal axis regulation and organization in urban and rural song sparrows

Urban habitats present animals with persistent disturbances and acute stressors not present in rural habitats or present at significantly lower levels. Differences in the glucocorticoid stress response could underlie colonization of these novel habitats. Despite urban habitats characterization as more stressful, previous comparisons of urban and rural birds have failed to find consistent differences in baseline and stress induced glucocorticoid levels. Another aspect of glucocorticoid regulation that could underlie an animal's ability to inhabit novel habitats, but has yet to be well examined, is more efficient termination of the glucocorticoid stress response which would allow birds in urban habitats to recover more quickly after a disturbance. The glucocorticoid stress response is terminated by negative feedback achieved primarily through their binding of receptors in the hippocampus and hypothalamus and subsequent decreased synthesis and release from the adrenals. We investigated if male song sparrows (Melospiza melodia) in urban habitats show more efficient termination of the glucocorticoid stress response than their rural counterparts using two approaches. First, we measured glucocorticoid receptor, mineralocorticoid receptor and 11β-HSD2 (an enzyme that inactivates corticosterone) mRNA expression in negative feedback targets of the brain (the hippocampus and hypothalamus) as a proxy measure of sensitivity to negative feedback. Second, we measured plasma corticosterone levels after standardized restraint and again following a challenge with the synthetic glucocorticoid, dexamethasone, as a means of assessing how quickly birds decreased glucocorticoid synthesis and release. Though there were no differences in the hypothalamus of urban and rural song sparrows, urban birds had lower glucocorticoid receptor and 11β-HSD2 mRNA expression in the hippocampus. Further, urban and rural birds had similar reductions in corticosterone following the dexamethasone challenge, suggesting that they do not differ in how quickly they decrease glucocorticoid synthesis and release. Thus, urban and rural song sparrows display similar termination of the glucocorticoid stress response even though urban birds have decreased hippocampal glucocorticoid receptor and 11β-HSD2 abundance.

11ß hydroxysteroid dehydrogenases regulate circulating glucocorticoids but not central gene expression

Regulation of glucocorticoids (GCs), important mediators of physiology and behavior at rest and during stress, is multi-faceted and dynamic. The 11ß hydroxysteroid dehydrogenases 11ß-HSD1 and 11ß-HSD2 catalyze the regeneration and inactivation of GCs, respectively, and provide peripheral and central control over GC actions in mammals. While these enzymes have only recently been investigated in just two songbird species, central expression patterns suggest that they may function differently in birds and mammals, and little is known about how peripheral expression regulates circulating GCs. In this study, we utilized the 11ß-HSD inhibitor carbenoxolone (CBX) to probe the functional effects of 11ß-HSD activity on circulating GCs and central GC-dependent gene expression in the adult zebra finch (Taeniopygia guttata). Peripheral CBX injection produced a marked increase in baseline GCs 60 min after injection, suggestive of a dominant role for 11ß-HSD2 in regulating circulating GCs. In the adult zebra finch brain, where 11ß-HSD2 but not 11ß-HSD1 is expressed, co-incubation of micro-dissected brain regions with CBX and stress-level GCs had no impact on expression of several GC-dependent genes. These results suggest that peripheral 11ß-HSD2 attenuates circulating GCs, whereas central 11ß-HSD2 has little impact on gene expression. Instead, rapid 11ß-HSD2-based regulation of local GC levels might fine-tune membrane GC actions in brain. These results provide new insights into the dynamics of GC secretion and action in this important model organism.

Effects of Maternal Chewing on Prenatal Stress-Induced Cognitive Impairments in the Offspring via Multiple Molecular Pathways

We aimed to investigate the effects of maternal chewing on prenatal stress-induced cognitive impairments in the offspring and to explore the molecular pathways of maternal chewing in a mice model. Maternal chewing ameliorated spatial learning impairments in the offspring in a Morris water maze test. Immunohistochemistry and Western blot findings revealed that maternal chewing alleviated hippocampal neurogenesis impairment and increased the expression of hippocampal brain-derived neurotrophic factor in the offspring. In addition, maternal chewing increased the expression of glucocorticoid receptor (GR) and 11β-hydroxysteroid dehydrogenase isozyme 2 (11β-HSD2) and decreased the expression of 11β-HSD1 in the placenta, thereby attenuating the increase of glucocorticoid in the offspring. Furthermore, maternal chewing increased the expression of 11β-HSD2, FK506-binding protein 51 (FKBP51) and FKBP52 and decreased the expression of 11β-HSD1, thereby increasing hippocampal nuclear GR level. In addition, maternal chewing attenuated the increase in expression of DNMT1 and DNMT3a and the decrease in expression of histone H3 methylation at lysine 4, 9, 27 and histone H3 acetylation at lysine 9 induced by prenatal stress in the offspring. Our findings suggest that maternal chewing could ameliorate prenatal stress-induced cognitive impairments in the offspring at least in part by protecting placenta barrier function, alleviating hippocampal nuclear GR transport impairment and increasing the hippocampal brain-derived neurotrophic factor (BDNF) level.

The stressed brain: regional and stress-related corticosterone and stress-regulated gene expression in the adult zebra finch (Taeniopygia guttata)

Glucocorticoids (CORT) are well-known as important regulators of behaviour and cognition at basal levels and under stress. However, the precise mechanisms governing CORT action and functional outcomes of this action in the brain remain unclear, particularly in model systems other than rodents. In the present study, we investigated the dynamics of CORT regulation in the zebra finch, an important model system for vocal learning, neuroplasticity and cognition. We tested the hypothesis that CORT is locally regulated in the zebra finch brain by quantifying regional and stress-related variation in total CORT across brain regions. In addition, we used an ex vivo slice culture system to test whether CORT regulates target gene expression uniquely in discrete regions of the brain. We documented a robust increase in brain CORT across regions after 30 minutes of restraint stress but, interestingly, baseline and stress-induced CORT levels varied between regions. In addition, CORT treatment of brain slice cultures differentially affected expression of three CORT target genes: it up-regulated expression of FKBP5 in most regions and SGK1 in the hypothalamus only, whereas GILZ was unaffected by CORT treatment across all brain regions investigated. The specific mechanisms producing regional variation in CORT and CORT-dependent downstream gene expression remain unknown, although these data provide additional support for the hypothesis that the songbird brain employs regulatory mechanisms that result in precise control over the influence of CORT on glucocorticoid-sensitive neural circuits.

Tissue-specific expression of 11β-HSD and its effects on plasma corticosterone during the stress response

The hypothalamic-pituitary-adrenal (HPA) axis is under complex regulatory control at multiple levels. Enzymatic regulation plays an important role in both circulating levels of glucocorticoids and target tissue exposure. Three key enzyme pathways are responsible for the immediate control of glucocorticoids. De novo synthesis of glucocorticoid from cholesterol involves a multistep enzymatic cascade. This cascade terminates with 11β-hydroxylase, responsible for the final conversion of 11-deoxy precursors into active glucocorticoids. Additionally, 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) controls regeneration of glucocorticoids from inactive metabolites, providing a secondary source of active glucocorticoids. Localized inactivation of glucocorticoids is under the control of Type 2 11β-HSD (11β-HSD2). The function of these enzymes is largely unexplored in wild species, particularly songbirds. Here, we investigated the contribution of both clearance and generation of glucocorticoids to regulation of the hormonal stress response via the use of pharmacological antagonists. Additionally, we mapped 11β-HSD gene expression. We found 11β-HSD1 primarily in liver, kidney and adrenal glands, although it was detectable across all tissue types. 11β-HSD2 was predominately expressed in the adrenal glands and kidney with moderate gonadal and liver expression. Inhibition of glucocorticoid generation by metyrapone was found to decrease levels peripherally, while both peripheral and central administration of the 11β-HSD2 inhibitor DETC resulted in elevated concentrations of corticosterone. These data suggest that during the stress response, peripheral antagonism of the 11β-HSD system has a greater impact on circulating glucocorticoid levels than central control. Further studies should aim to elucidate the respective roles of the 11β-HSD and 11β-hydroxylase enzymes.

Phenotypic flexibility of glucocorticoid signaling in skeletal muscles of a songbird preparing to migrate

Glucocorticoids are commonly associated with responses to stress, but other important functions include homeostatic regulation, energy metabolism and tissue remodeling. At low circulating levels, glucocorticoids bind to high-affinity mineralocorticoid receptors (MR) to activate tissue repair and homeostasis (anabolic pathways), whereas at elevated levels, glucocorticoids bind to glucocorticoid receptors (GR) to activate catabolic pathways. Long distance migrations, such as those performed by Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), require modification of anatomy, physiology and behavior. Plasma corticosterone (CORT) increases in association with impending departure and flight and may promote muscle-specific anabolic states. To test this idea, we explored glucocorticoid signaling in the pectoralis (flight) and gastrocnemius (leg) muscles of male sparrows on the wintering grounds at three stages leading up to spring departure: winter (February), pre-nuptial molt (March), and pre-departure (April). CORT was detected in plasma and in both muscles, but measures of CORT signaling differed across muscles and stages. Expression of 11β-hydroxysteroid dehydrogenase (11β-HSD) Type 2 (inactivates CORT) increased in the pectoralis at pre-departure, whereas 11β-HSD Type 1 (regenerates CORT) did not change. Neither of the two 11β-HSD isoforms was detectable in the gastrocnemius. Expression of MR, but not GR, was elevated in the pectoralis at pre-departure, while only GR expression was elevated at pre-nuptial molt in gastrocnemius. These data suggest that anabolic functions predominate in the pectoralis only while catabolic activity is undetected in either muscle at pre-departure.

Measurement of 11-dehydrocorticosterone in mice, rats and songbirds: Effects of age, sex and stress

Glucocorticoids (GCs) are secreted into the blood by the adrenal glands and are also locally-produced by organs such as the lymphoid organs (bone marrow, thymus, and spleen). Corticosterone is the primary circulating GC in many species, including mice, rats and birds. Within lymphoid organs, corticosterone can be locally produced from the inactive metabolite, 11-dehydrocorticosterone (DHC). However, very little is known about endogenous DHC levels, and no immunoassays are currently available to measure DHC. Here, we developed an easy-to-use and inexpensive immunoassay to measure DHC that is accurate, precise, sensitive, and specific. The DHC immunoassay was validated in multiple ways, including comparison with a mass spectrometry assay. After assay validations, we demonstrated the usefulness of this immunoassay by measuring DHC (and corticosterone) in mice, rats and song sparrows. Overall, corticosterone levels were higher than DHC levels across species. In Study 1, using mice, we measured steroids in whole blood and lymphoid organs at postnatal day (PND) 5, PND23, and PND90. Corticosterone and DHC showed distinct tissue-specific patterns across development. In Studies 2 and 3, we measured circulating corticosterone and DHC in adult rats and song sparrows, before and after restraint stress. In rats and song sparrows, restraint stress rapidly increased circulating levels of both steroids. This novel DHC immunoassay revealed major changes in DHC concentrations during development and in response to stress, which have important implications for understanding GC physiology, effects of stress on immune function, and regulation of local GC levels.

The brain development of infants with intrauterine growth restriction: role of glucocorticoids

Brain injury is a serious complication of intrauterine growth restriction (IUGR), but the exact mechanism remains unclear. While glucocorticoids (GCs) play an important role in intrauterine growth and development, GCs also have a damaging effect on microvascular endothelial cells. Moreover, intrauterine adverse environments lead to fetal growth restriction and the hypothalamus-pituitary-adrenal (HPA) axis resetting. In addition, chronic stress can cause a decrease in the number and volume of astrocytes in the hippocampus and glial cells play an important role in neuronal differentiation. Therefore, it is speculated that the effect of GCs on cerebral neurovascular units under chronic intrauterine stimulation is an important mechanism leading to brain injury in infants with growth restrictions.

Preparing to migrate: expression of androgen signaling molecules and insulin-like growth factor-1 in skeletal muscles of Gambel's white-crowned sparrows

Migratory birds, including Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), exhibit profound modifications of skeletal muscles prior to migration, notably hypertrophy of the pectoralis muscle required for powered flight. Muscle growth may be influenced by anabolic effects of androgens; however, prior to spring departure, circulating androgens are low in sparrows. A seasonal increase in local androgen signaling may occur within muscle to promote remodeling. We measured morphological parameters, plasma and tissue levels of testosterone, as well as mRNA expression levels of androgen receptor, 5α-reductase (converts testosterone to 5α-dihydrotestosterone), and the androgen-dependent myotrophic factor insulin-like growth factor-1. We studied the pectoralis muscle as well as the gastrocnemius (leg) muscle of male sparrows across three stages on the wintering grounds: winter (February), pre-nuptial molt (March), and pre-departure (April). Testosterone levels were low, but detectable, in plasma and muscles at all three stages. Androgen receptor mRNA and 5α-reductase Type 1 mRNA increased at pre-departure, but did so in both muscles. Notably, mRNA levels of insulin-like growth factor-1, an androgen-dependent gene critical for muscle remodeling, increased at pre-departure in the pectoralis but decreased in the gastrocnemius. Taken together, these data suggest a site-specific molecular basis for muscle remodeling that may serve to enable long-distance flight.

Seasonally sympatric but allochronic: differential expression of hypothalamic genes in a songbird during gonadal development

Allochrony, the mismatch of reproductive schedules, is one mechanism that can mediate sympatric speciation and diversification. In songbirds, the transition into breeding condition and gonadal growth is regulated by the hypothalamic-pituitary-gonadal (HPG) axis at multiple levels. We investigated whether the difference in reproductive timing between two seasonally sympatric subspecies of dark-eyed juncos (Junco hyemalis) was related to gene expression along the HPG axis. During the sympatric pre-breeding stage, we measured hypothalamic and testicular mRNA expression of candidate genes via qPCR in captive male juncos. For hypothalamic mRNA, we found our earlier breeding subspecies had increased expression of gonadotropin-releasing hormone (GnRH) and decreased expression of androgen receptor, oestrogen receptor alpha and mineralocorticoid receptor (MR). Subspecies did not differ in expression of hypothalamic gonadotropin-inhibitory hormone (GnIH) and glucocorticoid receptor (GR). While our earlier breeding subspecies had higher mRNA expression of testicular GR, subspecies did not differ in testicular luteinizing hormone receptor, follicle-stimulating hormone receptor or MR mRNA expression levels. Our findings indicate increased GnRH production and decreased hypothalamic sensitivity to sex steroid negative feedback as factors promoting differences in the timing of gonadal recrudescence between recently diverged populations. Differential gene expression along the HPG axis may facilitate species diversification under seasonal sympatry.