Act locally and think globally: intracerebral testosterone implants induce seasonal-like growth of adult avian song control circuits

Male and female red-cheeked cordon bleus sing similar yet individualistic songs

Birdsong is an excellent system for studying complex vocal signaling in both males and females. Historically, most research in captivity has focused only on male song. This has left a gap in our understanding of the environmental, neuroendocrine, and mechanistic control of female song. Here, we report the overall acoustic features, repertoire, and stereotypy of both male and female Red-Cheeked Cordon Bleus (Uraeginthus bengalus) (RCCBs) songs in the lab. We found few sex differences in the acoustic structure, song repertoire, and song stereotypy of RCCBs. Both sexes had similar song entropy, peak frequency, and duration. Additionally, individuals of both sexes sang only a single song type each and had similar levels of song and syllable stereotypy. However, we did find that female RCCBs had higher song bandwidth but lower syllable repertoires. Finally, and most strikingly, we found highly individualistic songs in RCCBs. Each individual produced a stereotyped and unique song with no birds sharing song types and very few syllable types being shared between birds of either sex. We propose that RCCBs represent a promising species for future investigations of the acoustic sex differences in song in a lab environment, and also for understanding the evolutionary driving forces behind individualistic songs.

Vernal growth of vocal control nucleus Area X, but not HVC, precedes gonadal recrudescence in wild black-capped chickadees (Poecile atricapillus)

In temperate-zone songbirds, the neuroanatomical changes which occur in advance of breeding, including the growth of nuclei of the vocal control system, are believed to occur downstream of gonadal recrudescence. However, evidence from wild birds is mixed. Here, we captured black-capped chickadees from the wild in early spring (March-April), summer (August-September), and winter (December-January); in addition to measuring the volumes of two vocal control nuclei (Area X and HVC), we also quantified two indicators of reproductive state (gonads and circulating gonadal steroids). Most birds captured in early spring had regressed gonads and low levels of circulating gonadal steroids, indicating these birds were not yet in full breeding condition. However, these early spring birds still had a significantly larger Area X than winter birds, while HVC did not differ in size across groups. Using data from a previously published seasonal study of black-capped chickadees (Phillmore et al., Developmental Neurobiology, 2015;75:203-216), we then compared Area X and HVC volumes from our early spring group to a breeding group of chickadees captured 3-4 weeks later in the spring. While Area X volume did not differ between the studies, breeding males in Phillmore et al. (2015) had a significantly larger HVC. Taken together, this suggests that the vernal growth of Area X occurs ahead of HVC in black-capped chickadees, and that the overall vernal changes in the vocal control system occur at least partially in advance of the breeding-associated upregulation of the hypothalamic-pituitary-gonadal axis.

Sex differences in seasonal brain plasticity and the neuroendocrine regulation of vocal behavior in songbirds

Birdsong is controlled in part by a discrete network of interconnected brain nuclei regulated in turn by steroid hormones and environmental stimuli. This complex interaction results in neural changes that occur seasonally as the environment varies (e.g., photoperiod, food/water availability, etc.). Variation in environment, vocal behavior, and neuroendocrine control has been primarily studied in male songbirds in both laboratory studies of captive birds and field studies of wild caught birds. The bias toward studying seasonality in the neuroendocrine regulation of song in male birds comes from a historic focus on sexually selected male behaviors. In fact, given that male song is often loud and accompanied by somewhat extravagant courtship behaviors, female song has long been overlooked. To compound this bias, the primary model songbird species for studies in the lab, zebra finches (Taeniopygia guttata) and canaries (Serinus canaria), exhibit little or no female song. Therefore, understanding the degree of variation and neuroendocrine control of seasonality in female songbirds is a major gap in our knowledge. In this review, we discuss the importance of studying sex differences in seasonal plasticity and the song control system. Specifically, we discuss sex differences in 1) the neuroanatomy of the song control system, 2) the distribution of receptors for androgens and estrogens and 3) the seasonal neuroplasticity of the hypothalamo-pituitary-gonadal axis as well as in the neural and cellular mechanisms mediating song system changes. We also discuss how these neuroendocrine mechanisms drive sex differences in seasonal behavior. Finally, we highlight specific gaps in our knowledge and suggest experiments critical for filling these gaps.

The estrous cycle and 17β-estradiol modulate the electrophysiological properties of rat nucleus accumbens core medium spiny neurons

The nucleus accumbens core is a key nexus within the mammalian brain for integrating the premotor and limbic systems and regulating important cognitive functions such as motivated behaviors. Nucleus accumbens core functions show sex differences and are sensitive to the presence of hormones such as 17β-estradiol (estradiol) in normal and pathological contexts. The primary neuron type of the nucleus accumbens core, the medium spiny neuron (MSN), exhibits sex differences in both intrinsic excitability and glutamatergic excitatory synapse electrophysiological properties. Here, we provide a review of recent literature showing how estradiol modulates rat nucleus accumbens core MSN electrophysiology within the context of the estrous cycle. We review the changes in MSN electrophysiological properties across the estrous cycle and how these changes can be mimicked in response to exogenous estradiol exposure. We discuss in detail recent findings regarding how acute estradiol exposure rapidly modulates excitatory synapse properties in nucleus accumbens core but not caudate-putamen MSNs, which mirror the natural changes seen across estrous cycle phases. These recent insights demonstrate the strong impact of sex-specific estradiol action upon nucleus accumbens core neuron electrophysiology.

Proposing a neural framework for the evolution of elaborate courtship displays

In many vertebrates, courtship occurs through the performance of elaborate behavioral displays that are as spectacular as they are complex. The question of how sexual selection acts upon these animals' neuromuscular systems to transform a repertoire of pre-existing movements into such remarkable (if not unusual) display routines has received relatively little research attention. This is a surprising gap in knowledge, given that unraveling this extraordinary process is central to understanding the evolution of behavioral diversity and its neural control. In many vertebrates, courtship displays often push the limits of neuromuscular performance, and often in a ritualized manner. These displays can range from songs that require rapid switching between two independently controlled 'voice boxes' to precisely choreographed acrobatics. Here, we propose a framework for thinking about how the brain might not only control these displays, but also shape their evolution. Our framework focuses specifically on a major midbrain area, which we view as a likely important node in the orchestration of the complex neural control of behavior used in the courtship process. This area is the periaqueductal grey (PAG), as studies suggest that it is both necessary and sufficient for the production of many instinctive survival behaviors, including courtship vocalizations. Thus, we speculate about why the PAG, as well as its key inputs, might serve as targets of sexual selection for display behavior. In doing so, we attempt to combine core ideas about the neural control of behavior with principles of display evolution. Our intent is to spur research in this area and bring together neurobiologists and behavioral ecologists to more fully understand the role that the brain might play in behavioral innovation and diversification.

Perinatal activation of ER and ER but not GPER-1 masculinizes female rat caudate-putamen medium spiny neuron electrophysiological properties

Exposure to steroid sex hormones such as 17β-estradiol (estradiol) during early life potentially permanently masculinize neuron electrophysiological phenotype. In rodents, one crucial component of this developmental process occurs in males, with estradiol aromatized in the brain from testes-sourced testosterone. However, it is unknown whether most neuron electrophysiological phenotypes are altered by this early masculinization process, including medium spiny neurons (MSNs) of the rat caudate-putamen. MSNs are the predominant and primary output neurons of the caudate-putamen and exhibit increased intrinsic excitability in females compared to males. Here, we hypothesize that since perinatal estradiol exposure occurs in males, then a comparable exposure in females to estradiol or its receptor agonists would be sufficient to induce masculinization. To test this hypothesis, we injected perinatal female rats with estradiol or its receptor agonists and then later assessed MSN electrophysiology. Female and male rats on postnatal day 0 and 1 were systemically injected with either vehicle, estradiol, the estrogen receptor (ER)α agonist PPT, the ERβ agonist DPN, or the G-protein-coupled receptor 1 (GPER-1) agonist G1. On postnatal days 19 ± 2, MSN electrophysiological properties were assessed using whole cell patch clamp recordings. Estradiol exposure abolished increased intrinsic excitability in female compared to male MSNs. Exposure to either an ERα or ERβ agonist masculinized female MSN evoked action potential firing rate properties, whereas exposure to an ERβ agonist masculinized female MSN inward rectification properties. Exposure to ER agonists minimally impacted male MSN electrophysiological properties. These findings indicate that perinatal estradiol exposure masculinizes MSN electrophysiological phenotype via activation of ERα and ERβ.NEW & NOTEWORTHY This study is the first to demonstrate that estradiol and estrogen receptor α and β stimulation during early development sexually differentiates the electrophysiological properties of caudate-putamen medium spiny neurons, the primary output neuron of the striatal regions. Overall, this evidence provides new insight into the neuroendocrine mechanism by which caudate-putamen neuron electrophysiology is sexually differentiated and demonstrates the powerful action of early hormone exposure upon individual neuron electrophysiology.

Sex differences and similarities in the neural circuit regulating song and other reproductive behaviors in songbirds

In the 1970s, Nottebohm and Arnold reported marked male-biased sex differences in the volume of three song control nuclei in songbirds. Subsequently a series of studies on several songbird species suggested that there is a positive correlation between the degree to which there is a sex difference in the volume of these song control nuclei and in song behavior. This correlation has been questioned in recent years. Furthermore, it has become clear that the song circuit is fully integrated into a more comprehensive neural circuit that regulates multiple courtship and reproductive behaviors including song. Sex differences in songbirds should be evaluated in the context of the full complement of behaviors produced by both sexes in relation to reproduction and based on the entire circuit in order to understand the functional significance of variation between males and females in brain and behavior. Variation in brain and behavior exhibited among living songbird species provides an excellent opportunity to understand the functional significance of sex differences related to social behaviors.

How does testosterone act to regulate a multifaceted adaptive response? Lessons from studies of the avian song system

In male songbirds, song functions to attract a mate or to defend a territory; it is therefore often produced in the context of reproduction. Testosterone of gonadal origin increases during the reproductive phase of the annual cycle and significantly enhances song production, as well as song development, via effects on song crystallisation. The neural control of birdsong production and learning is highly modular. We implanted testosterone or androgen antagonists into specific brain regions or in the periphery of castrated male canaries and, in this way, identified how androgen signalling in specific locations regulates a variety of birdsong features. For example, castrated male canaries treated with testosterone in the preoptic area only and exposed to long days sing at high rates compared to castrated male canaries not treated with testosterone. However, these birds with testosterone in the preoptic area still produce songs with substantially lower song stereotypy and amplitude; these features are controlled by testosterone acting in the song control nuclei HVC and robust nucleus of the arcopallium. Specific aspects of the learned singing behaviour are thus regulated by androgens acting at multiple levels in the brain in a non-redundant fashion. The action of testosterone in the preoptic area is related to the hormonal regulation of the motivation to sing but not to various aspects of song performance. Multiple aspects of song quality are instead precisely regulated by steroids acting in distinct song control nuclei. Females exert a strong choice for specific features of male song in canaries and this choice is influenced by the endocrine state of the female. The female song system is also involved in song production, as well as song perception, although the specificity of this hormone action has not yet been investigated.

Characteristics of song, brain-anatomy and blood androgen levels in spontaneously singing female canaries

Females of many northern temperate songbird species sing sporadically. However, detailed descriptions of female song are rare. Here we report a detailed analysis of song in a small number of spontaneously-singing female domesticated canaries (Serinus canaria) under non-breeding, laboratory conditions in a large population of domesticated birds. In-depth analysis showed that these females sang rarely, and the spontaneous songs varied between and within birds over time. Furthermore, spontaneous female songs were distinct from songs of testosterone-induced singing female canaries and from songs of male canaries in both temporal and spectral features. Singing females had significantly elevated plasma androgen levels and a larger size of the major song controlling brain nuclei HVC (used as a proper name) and the robust nucleus of the arcopallium (RA) than non-singing females housed under similar conditions. The sporadically observed production of song and accompanying differences in brain anatomy in female canaries may thus depend on minute intraspecific differences in androgen levels.

Brain-Derived Neurotrophic Factor Has a Transsynaptic Trophic Effect on Neural Activity in an Adult Forebrain Circuit

While hormone-driven plasticity in the adult brain is well studied, the underlying cellular and molecular mechanisms are less well understood. One example of this is seasonal plasticity in the avian brain, where song nuclei exhibit hormonally driven changes in response to changing photoperiod and circulating sex steroid hormones. Hormone receptor activation in song nucleus HVC (proper name) elicits a robust change in activity in target nucleus RA (robust nucleus of the arcopallium), but the molecular signal responsible for this is unknown. This study addressed whether brain-derived neurotrophic factor (BDNF) mediates a transsynaptic effect from HVC to RA in male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii). In situ hybridization confirmed an increase in BDNF expression in HVC neurons of birds switched to a long-day (LD) photoperiod plus systemically elevated testosterone (T) levels, compared with short-day (SD) conditions. BDNF expression was virtually absent in RA neurons of SD birds, increasing to barely detectable levels in a small subset of cells in LD+T birds. Infusion of BDNF protein adjacent to the RA of SD birds caused an increase in the spontaneous neuron firing rate. Conversely, the infusion of ANA12, a specific antagonist of the tyrosine-related kinase B (TrkB) for BDNF, prevented the increase in RA neuron firing rate in LD+T birds. These results indicate that BDNF is sufficient, and TrkB receptor activation is necessary, for the transsynaptic trophic effect exerted by HVC on RA. The dramatic change in the activity of RA neurons during the breeding season provides a clear example of transsynaptic BDNF effects in the adult brain in a functionally relevant circuit.SIGNIFICANCE STATEMENT Sex steroid hormones drive changes in brain circuits in all vertebrates, both within specific neurons and on their synaptic targets. Such changes can lead to profound changes in behavior, but little is known about the precise molecular mechanisms that underlie this process. We addressed this question in a seasonally breeding songbird and found that the trophic effects of one forebrain song nucleus on its target are mediated transsynaptically by the neurotrophin BDNF. This suggests that, in addition to their role in development, neurotrophins have critical roles in adult brain plasticity.

Rapamycin blocks the neuroprotective effects of sex steroids in the adult birdsong system

In adult songbirds, the telencephalic song nucleus HVC and its efferent target RA undergo pronounced seasonal changes in morphology. In breeding birds, there are increases in HVC volume and total neuron number, and RA neuronal soma area compared to nonbreeding birds. At the end of breeding, HVC neurons die through caspase-dependent apoptosis and thus, RA neuron size decreases. Changes in HVC and RA are driven by seasonal changes in circulating testosterone (T) levels. Infusing T, or its metabolites 5α-dihydrotestosterone (DHT) and 17 β-estradiol (E2), intracerebrally into HVC (but not RA) protects HVC neurons from death, and RA neuron size, in nonbreeding birds. The phosphoinositide 3-kinase (PI3K)-Akt (a serine/threonine kinase)-mechanistic target of rapamycin (mTOR) signaling pathway is a point of convergence for neuroprotective effects of sex steroids and other trophic factors. We asked if mTOR activation is necessary for the protective effect of hormones in HVC and RA of adult male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii). We transferred sparrows from breeding to nonbreeding hormonal and photoperiod conditions to induce regression of HVC neurons by cell death and decrease of RA neuron size. We infused either DHT + E2, DHT + E2 plus the mTOR inhibitor rapamycin, or vehicle alone in HVC. Infusion of DHT + E2 protected both HVC and RA neurons. Coinfusion of rapamycin with DHT + E2, however, blocked the protective effect of hormones on HVC volume and neuron number, and RA neuron size. These results suggest that activation of mTOR is an essential downstream step in the neuroprotective cascade initiated by sex steroid hormones in the forebrain.

Estradiol rapidly modulates excitatory synapse properties in a sex- and region-specific manner in rat nucleus accumbens core and caudate-putamen

Estradiol acutely facilitates sex differences in striatum-dependent behaviors. However, little is understood regarding the underlying mechanism. In striatal regions in adult rodents, estrogen receptors feature exclusively extranuclear expression, suggesting that estradiol rapidly modulates striatal neurons. We tested the hypothesis that estradiol rapidly modulates excitatory synapse properties onto medium spiny neurons (MSNs) of two striatal regions, the nucleus accumbens core and caudate-putamen in adult female and male rats. We predicted there would be sex-specific differences in pre- and postsynaptic locus and sensitivity. We further analyzed whether MSN intrinsic properties are predictive of estrogen sensitivity. Estradiol exhibited sex-specific acute effects in the nucleus accumbens core: miniature excitatory postsynaptic current (mEPSC) frequency robustly decreased in response to estradiol in female MSNs, and mEPSC amplitude moderately increased in response to estradiol in both male and female MSNs. This increase in mEPSC amplitude is associated with MSNs featuring increased intrinsic excitability. No MSN intrinsic electrical property associated with changes in mEPSC frequency. Estradiol did not acutely modulate mEPSC properties in the caudate-putamen of either sex. This is the first demonstration of acute estradiol action on MSN excitatory synapse function. This demonstration of sex and striatal region-specific acute estradiol neuromodulation revises our understanding of sex hormone action on striatal physiology and resulting behaviors.NEW & NOTEWORTHY This study is the first to demonstrate rapid estradiol neuromodulation of glutamatergic signaling on medium spiny neurons (MSNs), the major output neuron of the striatum. These findings emphasize that sex is a significant biological variable both in MSN sensitivity to estradiol and in pre- and postsynaptic mechanisms of glutamatergic signaling. MSNs in different regions exhibit diverse responses to estradiol. Sex- and region-specific estradiol-induced changes to excitatory signaling on MSNs explain sex differences partially underlying striatum-mediated behaviors and diseases.

Accelerated redevelopment of vocal skills is preceded by lasting reorganization of the song motor circuitry

Complex motor skills take considerable time and practice to learn. Without continued practice the level of skill performance quickly degrades, posing a problem for the timely utilization of skilled motor behaviors. Here we quantified the recurring development of vocal motor skills and the accompanying changes in synaptic connectivity in the brain of a songbird, while manipulating skill performance by consecutively administrating and withdrawing testosterone. We demonstrate that a songbird with prior singing experience can significantly accelerate the re-acquisition of vocal performance. We further demonstrate that an increase in vocal performance is accompanied by a pronounced synaptic pruning in the forebrain vocal motor area HVC, a reduction that is not reversed when birds stop singing. These results provide evidence that lasting synaptic changes in the motor circuitry are associated with the savings of motor skills, enabling a rapid recovery of motor performance under environmental time constraints.

Tales of testosterone: Advancing our understanding of environmental endocrinology through studies of neotropical birds

Studies of birds have greatly advanced our understanding of how testosterone modulates complex phenotypes, specifically its role in mediating male reproductive and associated behaviors. Yet most of the foundational studies have been limited to northern latitude breeding species despite the fact that they represent only a small fraction of worldwide avian diversity. In contrast, phylogenetic, life-history, and mating system diversity all reach their apex in neotropical avifauna and yet these birds, along with more southern latitude species, remain very poorly understood from an endocrine perspective. Despite the relatively limited previous work on taxa breeding in Central and South America, empirical findings have had a disproportionately large impact on our understanding of testosterone's role in everything from geographic variation to behavioral roles and neuroplasticity. Here, we synthesize how studies of neotropical breeding avifauna have advanced our understanding of how testosterone's actions can and are associated with the broad patterns of phenotypic diversity that we see in birds. In addition, we outline how these studies can be used individually or in a comparative context to address fundamental questions about the environmental endocrinology of testosterone and to understand the diversity of roles that testosterone plays in mediating behavioral variation, reproductive strategies, and associated life-history trade-offs.

The regulation of birdsong by testosterone: Multiple time-scales and multiple sites of action

Contribution to Special Issue on Fast effects of steroids. Sex steroid hormones act during early development to shape the circuitry upon which these same hormones act in adulthood to control behavioral responses to various stimuli. The "organizational" vs. "activational" distinction was proposed to explain this temporal difference in hormone action. In both of these cases steroids were thought to act genomically over a time-scale of days to weeks. However, sex steroids can affect behavior over short (e.g., seconds or minutes) time-scales. Here, we discuss how testosterone controls birdsong via actions at different sites and over different time-scales, with an emphasis on this process in canaries (Serinus canaria). Our work shows that testosterone in the medial preoptic nucleus regulates the motivation to sing, but not aspects of song performance. Instead, different aspects of song performance are regulated by long-term actions of testosterone in steroid-sensitive cortical-like brain regions and the syrinx, the avian vocal production organ. On the other hand, acute aromatase inhibition rapidly reduces the availability of estrogens and this reduction is correlated with reductions in the motivation to sing and song performance. Thus, testosterone and its estrogenic metabolites regulate distinct features of birdsong depending on the site and temporal window of action. The number of brain areas expressing androgen receptors is higher in species producing learned vocalization as compared to species that produce unlearned calls. An appealing scenario is that rapid effects of steroids in specific brain regions is a derived trait secondary to the widespread genomic effects of steroids in systems where steroids coordinate morphological, physiological, and behavioral traits.

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

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

Intracranial administration of the G-protein coupled estrogen receptor 1 antagonist, G-15, selectively affects dimorphic characteristics of the song system in zebra finches (Taeniopygia guttata)

In zebra finches (Taeniopygia guttata), estradiol contributes to sexual differentiation of the song system but the receptor(s) underlying its action are not exactly known. Whereas mRNA and/or protein for nuclear estrogen receptors ERα and ERβ are minimally expressed, G-protein coupled estrogen receptor 1 (GPER1) has a much greater distribution within neural song regions and the syrinx. At present, however, it is unclear if this receptor contributes to dimorphic development of the song system. To test this, the specific GPER1 antagonist, G-15, was intracranially administered to zebra finches for 25 days beginning on the day of hatching. In males, G-15 significantly decreased nuclear volumes of HVC and Area X. It also decreased the muscle fiber sizes of ventralis and dorsalis in the syrinx. In females, G-15 had no effect on measures within the brain, but did increase fiber sizes of both muscle groups. In sum, these data suggest that GPER1 can have selective and opposing influences on dimorphisms within the song system, but since not all features were affected additional factors are likely involved. © 2018 Wiley Periodicals, Inc. Develop Neurobiol, 2018.

Sex differences in brain-derived neurotrophic factor signaling: Functions and implications

Brain-derived neurotrophic factor (BDNF) regulates diverse processes such as neuronal survival, differentiation, and plasticity. Accumulating evidence suggests that molecular events that direct sexual differentiation of the brain interact with BDNF signaling pathways. This Mini-Review first examines potential hormonal and epigenetic mechanisms through which sex influences BDNF signaling. We then examine how sex-specific regulation of BDNF signaling supports the development and function of sexually dimorphic neural circuits that underlie male-specific genital reflexes in rats and song production in birds. Finally, we discuss the implications of sex differences in BDNF signaling for gender-biased presentation of neurological and psychiatric diseases such as Alzheimer's disease. Although this Mini-Review focuses on BDNF, we try to convey the general message that sex influences brain functions in complex ways and underscore the requirement for and challenge of expanding research on sex differences in neuroscience. © 2016 Wiley Periodicals, Inc.

Dissociable Effects on Birdsong of Androgen Signaling in Cortex-Like Brain Regions of Canaries

The neural basis of how learned vocalizations change during development and in adulthood represents a major challenge facing cognitive neuroscience. This plasticity in the degree to which learned vocalizations can change in both humans and songbirds is linked to the actions of sex steroid hormones during ontogeny but also in adulthood in the context of seasonal changes in birdsong. We investigated the role of steroid hormone signaling in the brain on distinct features of birdsong using adult male canaries (Serinus canaria), which show extensive seasonal vocal plasticity as adults. Specifically, we bilaterally implanted the potent androgen receptor antagonist flutamide in two key brain regions that control birdsong. We show that androgen signaling in the motor cortical-like brain region, the robust nucleus of the arcopallium (RA), controls syllable and trill bandwidth stereotypy, while not significantly affecting higher order features of song such syllable-type usage (i.e., how many times each syllable type is used) or syllable sequences. In contrast, androgen signaling in the premotor cortical-like brain region, HVC (proper name), controls song variability by increasing the variability of syllable-type usage and syllable sequences, while having no effect on syllable or trill bandwidth stereotypy. Other aspects of song, such as the duration of trills and the number of syllables per song, were also differentially affected by androgen signaling in HVC versus RA. These results implicate androgens in regulating distinct features of complex motor output in a precise and nonredundant manner.SIGNIFICANCE STATEMENT Vocal plasticity is linked to the actions of sex steroid hormones, but the precise mechanisms are unclear. We investigated this question in adult male canaries (Serinus canaria), which show extensive vocal plasticity throughout their life. We show that androgens in two cortex-like vocal control brain regions regulate distinct aspects of vocal plasticity. For example, in HVC (proper name), androgens regulate variability in syntax but not phonology, whereas androgens in the robust nucleus of the arcopallium (RA) regulate variability in phonology but not syntax. Temporal aspects of song were also differentially affected by androgen signaling in HVC versus RA. Thus, androgen signaling may reduce vocal plasticity by acting in a nonredundant and precise manner in the brain.

Different Seasonal Patterns in Song System Volume in Willow Tits and Great Tits

In most species of seasonally breeding songbirds studied to date, the brain areas that control singing (i.e. the song control system, SCS) are larger during the breeding season than at other times of the year. In the family of titmice and chickadees (Paridae), one species, the blue tit (Cyanistes caeruleus), shows the typical pattern of seasonal changes, while another species, the black-capped chickadee (Poecile atricapillus), shows, at best, very reduced seasonal changes in the SCS. To test whether this pattern holds up in the two Parid lineages to which these two species belong, and to rule out that the differences in seasonal patterns observed were due to differences in geography or laboratory, we compared the seasonal patterns in two song system nuclei volumes (HVC and Area X) in willow tits (Poecile montanus), closely related to black-capped chickadees, and in great tits (Parus major), more closely related to blue tits, from the same area around Oulu, Finland. Both species had larger gonads in spring than during the rest of the year. Great tit males had a larger HVC in spring than at other times of the year, but their Area X did not change in size. Willow tits showed no seasonal change in HVC or Area X size, despite having much larger gonads in spring than the great tits. Our findings suggest that the song system of willow tits and their relatives may be involved in learning and producing nonsong social vocalizations. Since these vocalizations are used year-round, there may be a year-round demand on the song system. The great tit and blue tit HVC may change seasonally because the demand is only placed on the song system during the breeding season, since they only produce learned vocalizations during this time. We suggest that changes were not observed in Area X because its main role is in song learning, and there is evidence that great tits do not learn new songs after their first year of life. Further study is required to determine whether our hypothesis about the role of the song system in the learned, nonsong vocalizations of the willow tit and chickadee is correct, and to test our hypothesis about the role of Area X in the great tit song system.

Endocrine and social regulation of adult neurogenesis in songbirds

The identification of pronounced seasonal changes in the volume of avian song control nuclei stimulated the discovery of adult neurogenesis in songbirds as well as renewed studies in mammals including humans. Neurogenesis in songbirds is modulated by testosterone and other factors such as photoperiod, singing activity and social environment. Adult neurogenesis has been widely studied by labeling, with tritiated thymidine or its analog BrdU, cells duplicating their DNA in anticipation of their last mitotic division and following their fate as new neurons. New methods based on endogenous markers of cell cycling or of various stages of neuronal life have allowed for additional progress. In particular immunocytochemical visualization of the microtubule-associated protein doublecortin has provided an integrated view of neuronal replacement in the song control nucleus HVC. Multiple questions remain however concerning the specific steps in the neuronal life cycle that are modulated by various factors and the underlying cellular mechanisms.

Pleiotropic Control by Testosterone of a Learned Vocal Behavior and Its Underlying Neuroplasticity(1,2,3)

Steroid hormones coordinate multiple aspects of behavior and physiology. The same hormone often regulates different aspects of a single behavior and its underlying neuroplasticity. This pleiotropic regulation of behavior and physiology is not well understood. Here, we investigated the orchestration by testosterone (T) of birdsong and its neural substrate, the song control system. Male canaries were castrated and received stereotaxic implants filled with T in select brain areas. Implanting T solely in the medial preoptic nucleus (POM) increased the motivation to sing, but did not enhance aspects of song quality such as acoustic structure and stereotypy. In birds implanted with T solely in HVC (proper name), a key sensorimotor region of the song control system, little or no song was observed, similar to castrates that received no T implants of any sort. However, implanting T in HVC and POM simultaneously rescued all measures of song quality. Song amplitude, though, was still lower than what was observed in birds receiving peripheral T treatment. T in POM enhanced HVC volume bilaterally, likely due to activity-dependent changes resulting from an enhanced song rate. T directly in HVC, without increasing song rate, enhanced HVC volume on the ipsilateral side only. T in HVC enhanced the incorporation and recruitment of new neurons into this nucleus, while singing activity can independently influence the incorporation of new neurons into HVC. These results have broad implications for how steroid hormones integrate across different brain regions to coordinate complex social behaviors.

The dual action of estrogen hypothesis

Estradiol (E2) can act in the brain in a relatively fast manner (i.e., seconds to minutes) usually through signaling initiated at the cell membrane. Brain-derived E2 has thus been considered as another type of neurotransmitter. Recent work found that behaviors indicative of male sexual motivation are activated by estrogenic metabolites of testosterone (T) in a fast manner, while sexual performance (copulatory behavior per se) is regulated by brain E2 in a slower manner via nucleus-initiated actions. This functional division between these two types of action appears to generalize to other behavioral systems regulated by E2. We propose the dual action of estrogen hypothesis to explain this functional distinction between these two different modes of action.

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.

Seasonal change in the avian hippocampus

The hippocampus plays an important role in cognitive processes, including memory and spatial orientation, in birds. The hippocampus undergoes seasonal change in food-storing birds and brood parasites, there are changes in the hippocampus during breeding, and further changes occur in some species in association with migration. In food-storing birds, seasonal change in the hippocampus occurs in fall and winter when the cognitively demanding behaviour of caching and retrieving food occurs. The timing of annual change in the hippocampus of food-storing birds is quite variable, however, and appears not to be under photoperiod control. A variety of factors, including cognitive performance, exercise, and stress may all influence seasonal change in the avian hippocampus. The causal processes underlying seasonal change in the avian hippocampus have not been extensively examined and the more fully described hormonal influences on the mammalian hippocampus may provide hypotheses for investigating the control of hippocampal seasonality in birds.

Transsynaptic trophic effects of steroid hormones in an avian model of adult brain plasticity

The avian song control system provides an excellent model for studying transsynaptic trophic effects of steroid sex hormones. Seasonal changes in systemic testosterone (T) and its metabolites regulate plasticity of this system. Steroids interact with the neurotrophin brain-derived neurotrophic factor (BDNF) to influence cellular processes of plasticity in nucleus HVC of adult birds, including the addition of newborn neurons. This interaction may also occur transsynpatically; T increases the synthesis of BDNF in HVC, and BDNF protein is then released by HVC neurons on to postsynaptic cells in nucleus RA where it has trophic effects on activity and morphology. Androgen action on RA neurons increases their activity and this has a retrograde trophic effect on the addition of new neurons to HVC. The functional linkage of sex steroids to BDNF may be of adaptive value in regulating the trophic effects of the neurotrophin and coordinating circuit function in reproductively relevant contexts.

Contextual modulation of social and endocrine correlates of fitness: insights from the life history of a sex changing fish

Steroid hormones are critical regulators of reproductive life history, and the steroid sensitive traits (morphology, behavior, physiology) associated with particular life history stages can have substantial fitness consequences for an organism. Hormones, behavior and fitness are reciprocally associated and can be used in an integrative fashion to understand how the environment impacts organismal function. To address the fitness component, we highlight the importance of using reliable proxies of reproductive success when studying proximate regulation of reproductive phenotypes. To understand the mechanisms by which the endocrine system regulates phenotype, we discuss the use of particular endocrine proxies and the need for appropriate functional interpretation of each. Lastly, in any experimental paradigm, the responses of animals vary based on the subtle differences in environmental and social context and this must also be considered. We explore these different levels of analyses by focusing on the fascinating life history transitions exhibited by the bi-directionally hermaphroditic fish, Lythrypnus dalli. Sex changing fish are excellent models for providing a deeper understanding of the fitness consequences associated with behavioral and endocrine variation. We close by proposing that local regulation of steroids is one potential mechanism that allows for the expression of novel phenotypes that can be characteristic of specific life history stages. A comparative species approach will facilitate progress in understanding the diversity of mechanisms underlying the contextual regulation of phenotypes and their associated fitness correlates.

Sex steroid-independent effects of photostimulation on the song-control system of white-throated sparrows (Zonotrichia albicollis)

Brain nuclei within the song-control system of songbirds are seasonally plastic during adulthood. These nuclei are larger in birds exposed to long, spring-like days than short, winter-like days. There is overwhelming evidence that this effect is mediated by testosterone (T). However, castration studies have also demonstrated that photostimulation has gonad-independent effects on song-control system plasticity, but these studies rarely control for extra-gonadal sources of T. In this study, we used anti-androgen and anti-estrogen treatments in combination with castration to determine the sex steroid-independent effects of photostimulation on HVC size and doublecortin immunoreactivity in white-throated sparrows (Zonotrichia albicollis). Birds were kept on short days or photostimulated for 1 month. Photostimulated birds were intact, castrated and treated with anti-androgens and anti-estrogens, or castrated and treated with T. HVC volumes of photostimulated birds were significantly larger than short-day birds. HVC volume of castrated birds given anti-androgens/-estrogens was significantly larger than short-day birds, indicating a sex steroid-independent effect of photostimulation. Similar results were observed for RA. The number of migrating neurons (immunoreactive for doublecortin) in HVC did not differ between treatment groups. Our data support the view that photostimulation alone can drive song-control system nuclei growth, and that concurrent exposure to T potentiates this growth. Moreover, these effects do not appear dependent on modulation of neuron migration.

Dissociable effects of social context on song and doublecortin immunoreactivity in male canaries

Variation in environmental factors such as day length and social context greatly affects reproductive behavior and the brain areas that regulate these behaviors. One such behavior is song in songbirds, which males use to attract a mate during the breeding season. In these species the absence of a potential mate leads to an increase in the number of songs produced, while the presence of a mate greatly diminishes singing. Interestingly, although long days promote song behavior, producing song itself can promote the incorporation of new neurons in brain regions controlling song output. Social context can also affect such neuroplasticity in these song control nuclei. The goal of the present study was to investigate in canaries (Serinus canaria), a songbird species, how photoperiod and social context affect song and the incorporation of new neurons, as measured by the microtubule-associated protein doublecortin (DCX) in HVC, a key vocal production brain region of the song control system. We show that long days increased HVC size and singing activity. In addition, male canaries paired with a female for 2 weeks showed enhanced DCX-immunoreactivity in HVC relative to birds housed alone. Strikingly, however, paired males sang fewer songs that exhibited a reduction in acoustic features such as song complexity and energy, compared with birds housed alone, which sang prolifically. These results show that social presence plays a significant role in the regulation of neural and behavioral plasticity in songbirds and can exert these effects in opposition to what might be expected based on activity-induced neurogenesis.

Castration modulates singing patterns and electrophysiological properties of RA projection neurons in adult male zebra finches

Castration can change levels of plasma testosterone. Androgens such as testosterone play an important role in stabilizing birdsong. The robust nucleus of the arcopallium (RA) is an important premotor nucleus critical for singing. In this study, we investigated the effect of castration on singing patterns and electrophysiological properties of projection neurons (PNs) in the RA of adult male zebra finches. Adult male zebra finches were castrated and the changes in bird song assessed. We also recorded the electrophysiological changes from RA PNs using patch clamp recording. We found that the plasma levels of testosterone were significantly decreased, song syllable's entropy was increased and the similarity of motif was decreased after castration. Spontaneous and evoked firing rates, membrane time constants, and membrane capacitance of RA PNs in the castration group were lower than those of the control and the sham groups. Afterhyperpolarization AHP time to peak of spontaneous action potential (AP) was prolonged after castration.These findings suggest that castration decreases song stereotypy and excitability of RA PNs in male zebra finches.

Status-appropriate singing behavior, testosterone and androgen receptor immunolabeling in male European starlings (Sturnus vulgaris)

Vocalizations convey information about an individual's motivational, internal, and social status. As circumstances change, individuals respond by adjusting vocal behavior accordingly. In European starlings, a male that acquires a nest site socially dominates other males and dramatically increases courtship song. Although circulating testosterone is associated with social status and vocal production it is possible that steroid receptors fine-tune status-appropriate changes in behavior. Here we explored a possible role for androgen receptors. Male starlings that acquired nest sites produced high rates of courtship song. For a subset of males this occurred even in the absence of elevated circulating testosterone. Immunolabeling for androgen receptors (ARir) was highest in the medial preoptic nucleus (POM) in males with both a nest site and elevated testosterone. For HVC, ARir was higher in dominant males with high testosterone (males that sang longer songs) than dominant males with low testosterone (males that sang shorter songs). ARir in the dorsal medial portion of the nucleus intercollicularis (DM) was elevated in males with high testosterone irrespective of dominance status. Song bout length related positively to ARir in POM, HVC and DM, and testosterone concentrations related positively to ARir in POM and DM. Results suggest that the role of testosterone in vocal behavior differs across brain regions and support the hypothesis that testosterone in POM underlies motivation, testosterone in HVC relates to song quality, and testosterone in DM stimulates vocalizations. Our data also suggest that singing may influence AR independent of testosterone and that alternative androgen-independent pathways regulate status-appropriate singing behavior.

Differential effects of global versus local testosterone on singing behavior and its underlying neural substrate

Steroid hormones regulate multiple but distinct aspects of social behaviors. Testosterone (T) has multiple effects on learned courtship song in that it regulates both the motivation to sing in a particular social context as well as the quality of song produced. The neural substrate(s) where T acts to regulate the motivation to sing as opposed to other aspects of song has not been definitively characterized. We show here that T implants in the medial preoptic nucleus (POM) of castrated male canaries (Serinus canaria) increase song rate but do not enhance acoustic features such as song stereotypy compared with birds receiving peripheral T that can act globally throughout the brain. Strikingly, T action in the POM increased song control nuclei volume, consistent with the hypothesis that singing activity induces neuroplasticity in the song control system independent of T acting in these nuclei. When presented with a female canary, POM-T birds copulated at a rate comparable to birds receiving systemic T but produced fewer calls and songs in her presence. Thus, POM is a key site where T acts to activate copulation and increase song rate, an appetitive sexual behavior in songbirds, but T action in other areas of the brain or periphery (e.g., HVC, dopaminergic cell groups, or the syrinx) is required to enhance the quality of song (i.e., stereotypy) as well as regulate context-specific vocalizations. These results have broad implications for research concerning how steroids act at multiple brain loci to regulate distinct sociosexual behaviors and the associated neuroplasticity.

Peripheral androgen receptors sustain the acrobatics and fine motor skill of elaborate male courtship

Androgenic hormones regulate many aspects of animal social behavior, including the elaborate display routines on which many species rely for advertisement and competition. One way that this might occur is through peripheral effects of androgens, particularly on skeletal muscles that control complex movements and postures of the body and its limbs. However, the specific contribution of peripheral androgen-muscle interactions to the performance of elaborate behavioral displays in the natural world has never been examined. We study this issue in one of the only natural physiological models of animal acrobatics: the golden-collared manakin (Manacus vitellinus). In this tropical bird, males compete with each other and court females by producing firecracker-like wing- snaps and by rapidly dancing among saplings over the forest floor. To test how activation of peripheral androgen receptors (AR) influences this display, we treat reproductively active adult male birds with the peripherally selective antiandrogen bicalutamide (BICAL) and observe the effects of this manipulation on male display performance. We not only validate the peripheral specificity of BICAL in this species, but we also show that BICAL treatment reduces the frequency with which adult male birds perform their acrobatic display maneuvers and disrupts the overall structure and fine-scale patterning of these birds' main complex wing-snap sonation. In addition, this manipulation has no effect on the behavioral metrics associated with male motivation to display. Together, our findings help differentiate the various effects of peripheral and central AR on the performance of a complex sociosexual behavioral phenotype by indicating that peripheral AR can optimize the motor skills necessary for the production of an elaborate animal display.

With a little help from my friends: androgens tap BDNF signaling pathways to alter neural circuits

Gonadal androgens are critical for the development and maintenance of sexually dimorphic regions of the male nervous system, which is critical for male-specific behavior and physiological functioning. In rodents, the motoneurons of the spinal nucleus of the bulbocavernosus (SNB) provide a useful example of a neural system dependent on androgen. Unless rescued by perinatal androgens, the SNB motoneurons will undergo apoptotic cell death. In adulthood, SNB motoneurons remain dependent on androgen, as castration leads to somal atrophy and dendritic retraction. In a second vertebrate model, the zebra finch, androgens are critical for the development of several brain nuclei involved in song production in males. Androgen deprivation during a critical period during postnatal development disrupts song acquisition and dimorphic size-associated nuclei. Mechanisms by which androgens exert masculinizing effects in each model system remain elusive. Recent studies suggest that brain-derived neurotrophic factor (BDNF) may play a role in androgen-dependent masculinization and maintenance of both SNB motoneurons and song nuclei of birds. This review aims to summarize studies demonstrating that BDNF signaling via its tyrosine receptor kinase (TrkB) receptor may work cooperatively with androgens to maintain somal and dendritic morphology of SNB motoneurons. We further describe studies that suggest the cellular origin of BDNF is of particular importance in androgen-dependent regulation of SNB motoneurons. We review evidence that androgens and BDNF may synergistically influence song development and plasticity in bird species. Finally, we provide hypothetical models of mechanisms that may underlie androgen- and BDNF-dependent signaling pathways.

Testosterone and brain-derived neurotrophic factor interactions in the avian song control system

Interaction between steroid sex hormones and brain-derived neurotrophic factor (BDNF) is a common feature of vertebrate brain organization. The avian song control system provides an excellent model for studying such interactions in neural circuits that regulate song, a learned sensorimotor behavior that is often sexually dimorphic and restricted to reproductive contexts. Testosterone (T) and its steroid metabolites interact with BDNF during development of the song system and in adult plasticity, including the addition of newborn neurons to the pallial nucleus HVC and seasonal changes in structure and function of these circuits. T and BDNF interact locally within HVC to influence cell proliferation and survival. This interaction may also occur transsynpatically; T increases the synthesis of BDNF in HVC, and BDNF protein is then released on to postsynaptic cells in the robust nucleus of the arcopallium (RA) where it has trophic effects. The interaction between sex steroids and BDNF is an example of molecular exploitation, with the evolutionarily ancient steroid-receptor complex having been captured by the more recently evolved BDNF. The functional linkage of sex steroids to BDNF may be of adaptive value in regulating the trophic effects of the neurotrophin in sexually dimorphic and reproductively relevant contexts.

A species-specific view of song representation in a sensorimotor nucleus

Songbirds constitute a powerful model system for the investigation of how complex vocal communication sounds are represented and generated, offering a neural system in which the brain areas involved in auditory, motor and auditory-motor integration are well known. One brain area of considerable interest is the nucleus HVC. Neurons in the HVC respond vigorously to the presentation of the bird's own song and display song-related motor activity. In the present paper, we present a synthesis of neurophysiological studies performed in the HVC of one songbird species, the canary (Serinus canaria). These studies, by taking advantage of the singing behavior and song characteristics of the canary, have examined the neuronal representation of the bird's own song in the HVC. They suggest that breeding cues influence the degree of auditory selectivity of HVC neurons for the bird's own song over its time-reversed version, without affecting the contribution of spike timing to the information carried by these two song stimuli. Also, while HVC neurons are collectively more responsive to forward playback of the bird's own song than to its temporally or spectrally modified versions, some are more broadly tuned, with an auditory responsiveness that extends beyond the bird's own song. Lastly, because the HVC is also involved in song production, we discuss the peripheral control of song production, and suggest that interspecific variations in song production mechanisms could be exploited to improve our understanding of the functional role of the HVC in respiratory-vocal coordination.

Influence of testosterone metabolites on song-control system neuroplasticity during photostimulation in adult European starlings (Sturnus vulgaris)

The song-control system is a network of discrete nuclei in the songbird brain that controls the production and learning of birdsong and exhibits some of the best-studied neuroplasticity found in the adult brain. Photoperiodic growth of the song-control system during the breeding season is driven, at least in part, by the gonadal steroid testosterone. When acting on neural tissue, however, testosterone can be metabolized into 5α-dihydrotestosterone (DHT) or 17β-estradiol (E2), which activate different hormonal signaling pathways. By treating adult starlings with both testosterone metabolites and metabolite antagonists, we attempted to isolate the effects of androgen and estrogen treatment on neuroplasticity during photostimulation in male and female European starlings (Sturnus vulgaris). Photostimulation resulted in a large HVC volume typical of the breeding season in all treatments independent of hormone treatment. E2 had additional effects on HVC growth by reducing neuron density and enhancing early survival of new neurons recruited to HVC in females but did not significantly affect HVC volume. Conversely, DHT reduced the migration of new neurons, assessed by the expression of doublecortin, to HVC. DHT also increased syrinx mass and maintained RA (robust nucleus of the arcopallium) cytoarchitecture in the presence of aromatase inhibitors. In addition, we document the first evidence of sex-specific neuroplastic responses of the song-control system to androgens and estrogens. These findings suggest that the contributions of DHT and E2 signaling in songbird neuroplasticity may be regulated by photoperiod and that future studies should account for species and sex differences in the brain.

Androgens and estrogens synergistically regulate the expression of doublecortin and enhance neuronal recruitment in the song system of adult female canaries

Vocal control nuclei in songbirds display seasonal changes in volume that are regulated by testosterone (T) and its androgenic (5α-dihydrotestosterone; DHT) or estrogenic (17β-estradiol; E(2)) metabolites. In male canaries, T regulates expression of the microtubule-associated protein doublecortin (DCX), a marker of neurogenesis. We examined the effect of T and its two metabolites alone or in combination on DCX expression in adult female canaries. Treatment with T or with DHT+E(2) increased HVC volume and neuron numbers as well as the total numbers of fusiform (migrating) and round (differentiating) DCX neurons in the nucleus but generally not in adjacent areas. DHT or E(2) alone did not increase these measures but increased the density of fusiform DCX cells per section. Similar results were observed in area X, although some effects did not reach significance, presumably because plasticity in X is mediated transsynaptically and follows HVC changes with some delay. There was no effect of any treatment on the total number of neurons in area X, and no change in DCX cell densities was detected in the lateral magnocellular nucleus of the anterior nidopallium, nor in other parts of the nidopallium. DHT and E(2) by themselves thus increase density of DCX cells migrating through HVC but are not sufficient in isolation to induce the recruitment of these newborn neurons in the nucleus. These effects are generally not observed in the rest of the nidopallium, implying that steroids only act on the attraction and recruitment of new neurons in HVC without having any major effects on their production at the ventricle wall.

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

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

Neuroprotective effects of testosterone in a naturally occurring model of neurodegeneration in the adult avian song control system

Seasonal regression of the avian song control system, a series of discrete brain nuclei that regulate song learning and production, serves as a useful model for investigating the neuroprotective effects of steroids. In seasonally breeding male songbirds, the song control system regresses rapidly when males are transferred from breeding to nonbreeding physiological conditions. One nucleus in particular, the HVC, regresses in volume by 22% within days of castration and transfer to a nonbreeding photoperiod. This regression is mediated primarily by a 30% decrease in neuron number, a result of a caspase-dependent process of programmed cell death. Here we examine whether testosterone (T) can act locally in the brain to prevent seasonal-like neurodegeneration in HVC. We began to infuse T intracerebrally near HVC on one side of the brain in breeding-condition male white-crowned sparrows 2 days prior to T withdrawal and shifting them to short-day photoperiods. The birds were killed 3 or 7 days later. Local T infusion significantly protected ipsilateral HVC from volume regression and neuron loss. In addition, T infusion significantly reduced the number, density, and number/1,000 neurons of activated caspase-3 cells and cells positive for cleaved PARP, both markers for programmed cell death, in the ipsilateral HVC. T infusion near HVC also prevented regression of ipsilateral efferent targets of HVC neurons, including the volumes of robust nucleus of the arcopallium (RA) and Area X and the soma area and density of RA neurons. Thus T can act locally in the brain to have a neuroprotective effect and act transsynaptically to prevent regression of efferent nuclei.

Seasonal changes in androgen receptor mRNA in the brain of the white-crowned sparrow

In songbirds, neurons that regulate learned song behavior undergo extensive seasonal plasticity in their number and size in relation to the bird's reproductive status. Seasonal plasticity of these brain regions is primarily regulated by changes in circulating concentrations of testosterone. Androgen receptors are present in all of the major song nuclei, but it is unknown whether levels of androgen receptor mRNA in the telencephalic song regions HVC, the robust nucleus of the arcopallium, and the lateral magnocellular nucleus of the anterior nidopallium change as a function of season in white-crowned sparrows. To determine whether seasonal changes in levels of androgen receptor mRNA are specific to the song control system, we also measured levels of androgen receptor mRNA in a limbic nucleus, the lateral division of the bed nucleus of the stria terminalis (the lateral division of the bed nucleus of the stria terminalis). We found that levels of androgen receptor mRNA were higher in HVC and the lateral division of the bed nucleus of the stria terminalis of birds in the breeding condition compared with the nonbreeding condition; however, we observed no seasonal differences in levels of androgen receptor mRNA in either the robust nucleus of the arcopallium or the lateral magnocellular nucleus of the anterior nidopallium. These results are consistent with previous observations that seasonal plasticity of the song nuclei results from testosterone acting directly on HVC, which then exerts transsynaptic trophic effects on its efferent targets. The seasonal change in the expression of androgen receptor in HVC may be one component of the cellular mechanisms underlying androgenic effects on seasonal plasticity of the song control nuclei.

Song competition changes the brain and behavior of a male songbird

Males should adjust their behavior and its neural substrates according to the quality of competition that they assess by eavesdropping on other males' courtship signals. In European starlings (Sturnus vulgaris), females base mate choice, in part, on aspects of male song associated with its length, which positively correlates with the males' reproductive success, immunocompetence, age and ability to repel competing males. To determine how variation in the quality of male courtship song affects the brain and behavior of incidental male receivers, we exposed adult male starlings to either long or short songs periodically over 7 days, followed by 1 day of no song. We found no difference between groups in the length (i.e. quality) of songs that subjects produced during the experiment. However, compared with males exposed to short songs, those exposed to long songs sang more songs, exhibited more non-singing activity and, by the end of the experiment, weighed less and had a 30% larger robust nucleus of the arcopallium (RA), a forebrain nucleus that translates pre-motor signals into the appropriate combination of respiratory and syringeal activity. The change in RA volume was not entirely due to variation in song output, suggesting, for the first time, the possibility of acoustically driven plasticity in this motor nucleus. We hypothesize that such neuroplasticity helps prepare the individual for future song output tailored to the prevailing competitive environment.

Seasonal and hormonal modulation of neurotransmitter systems in the song control circuit

In the years following the discovery of the song system, it was realized that this specialized circuit controlling learned vocalizations in songbirds (a) constitutes a specific target for sex steroid hormone action and expresses androgen and (for some nuclei) estrogen receptors, (b) exhibits a chemical neuroanatomical pattern consisting in a differential expression of various neuropeptides and neurotransmitters receptors as compared to surrounding structures and (c) shows pronounced seasonal variations in volume and physiology based, at least in the case of HVC, on a seasonal change in neuron recruitment and survival. During the past 30 years numerous studies have investigated how seasonal changes, transduced largely but not exclusively through changes in sex steroid concentrations, affect singing frequency and quality by modulating the structure and activity of the song control circuit. These studies showed that testosterone or its metabolite estradiol, control seasonal variation in singing quality by a direct action on song control nuclei. These studies also gave rise to the hypothesis that the probability of song production in response to a given stimulus (i.e. its motivation) is controlled through effects on the medial preoptic area and on catecholaminergic cell groups that project to song control nuclei. Selective pharmacological manipulations confirmed that the noradrenergic system indeed plays a role in the control of singing behavior. More experimental work is, however, needed to identify specific genes related to neurotransmission that are regulated by steroids in functionally defined brain areas to enhance different aspects of song behavior.

Androgens enhance plasticity of an electric communication signal in female knifefish, Brachyhypopomus pinnicaudatus

Sex steroids were initially defined by their actions shaping sexually dimorphic behavioral patterns. More recently scientists have begun exploring the role of steroids in determining sex differences in behavioral plasticity. We investigated the role of androgens in potentiating circadian, pharmacological, and socially-induced plasticity in the amplitude and duration of electric organ discharges (EODs) of female gymnotiform fish. We first challenged female fish with injections of serotonin (5-HT) and adrenocorticotropic hormone (ACTH), and with social encounters with female and male conspecifics to characterize females' pre-implant responses to each treatment. Each individual was then implanted with a pellet containing dihydrotestosterone (DHT) concentrations of 0.0, 0.03, 0.1, 0.3, or 1.0 mg 10 g(-1) body weight. We then repeated all challenges and compared each female's pre- and post-implant responses. The highest implant dose enhanced EOD duration modulations in response to all challenge types, responses to male challenge were also greater at the second highest dose, and responses to ACTH challenge were enhanced in females receiving all but the smallest dose (and blank) implants. Alternatively, amplitude modulations were enhanced only during female challenges and only when females received the highest DHT dose. Our results highlight the differential regulation of EOD duration and amplitude, and suggest that DHT enhanced the intrinsic plasticity of the electrogenic cells that produce the EOD rather than modifying behavioral phenotypes. The relative failure of DHT to enhance EOD amplitude plasticity also implies that factors other than androgens are involved in regulating/promoting male-typical EOD circadian rhythms and waveform modulations displayed in social contexts.

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.

The role of neurotrophins in the seasonal-like growth of the avian song control system

The avian song control system undergoes pronounced seasonal plasticity in response to photoperiod and hormonal cues. The action of testosterone (T) and its metabolites in the song nucleus HVC is both necessary and sufficient to promote breeding season-like growth of its efferent nuclei RA (robust nucleus of the arcopallium) and Area X, suggesting that HVC may release a trophic factor such as brain-derived neurotrophic factor (BDNF) into RA and X. BDNF is involved in many forms of adult neural plasticity in other systems and is present in the avian song system. We used a combination of in situ hybridization and intracerebral infusions to test whether BDNF plays a role in the seasonal-like growth of the song system in adult male white-crowned sparrows. BDNF mRNA levels increased in HVC in response to breeding conditions, and BDNF infusion into RA was sufficient to promote breeding-like changes in somatic area and neuronal density. Expression of the mRNA for the Trk B receptor of BDNF, however, did not vary with seasonal conditions in either HVC or RA. Local blockade of BDNF activity in RA via infusion of Trk-Fc fusion proteins inhibited the response to breeding conditions. Our results indicate that BDNF is sufficient to promote the seasonal plasticity in somatic area and cell density in RA, although NT-3 may also contribute to this process, and suggest that HVC may be a presynaptic source of increased levels of BDNF in RA of breeding-condition birds.

Plastic and stable electrophysiological properties of adult avian forebrain song-control neurons across changing breeding conditions

Steroid sex hormones drive changes in the nervous system and behavior in many animal taxa, but integrating the former with the latter remains challenging. One useful model system for meeting this challenge is seasonally breeding songbirds. In these species, plasma testosterone levels rise and fall across the seasons, altering song behavior and causing dramatic growth and regression of the song-control system, a discrete set of nuclei that control song behavior. Whereas the cellular mechanisms underlying changes in nucleus volume have been studied as a model for neural growth and degeneration, it is unknown whether these changes in neural structure are accompanied by changes in electrophysiological properties other than spontaneous firing rate. Here we test the hypothesis that passive and active neuronal properties in the forebrain song-control nuclei HVC and RA change across breeding conditions. We exposed adult male Gambel's white-crowned sparrows to either short-day photoperiod or long-day photoperiod and systemic testosterone to simulate nonbreeding and breeding conditions, respectively. We made whole-cell recordings from RA and HVC neurons in acute brain slices. We found that RA projection neuron membrane time constant, capacitance, and evoked and spontaneous firing rates were all increased in the breeding condition; the measured electrophysiological properties of HVC interneurons and projection neurons were stable across breeding conditions. This combination of plastic and stable intrinsic properties could directly impact the song-control system's motor control across seasons, underlying changes in song stereotypy. These results provide a valuable framework for integrating how steroid hormones modulate cellular physiology to change behavior.