The canary androgen receptor mRNA is localized in the song control nuclei of the brain and is rapidly regulated by testosterone

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.

Neuroendocrine patterns underlying seasonal song and year-round territoriality in male black redstarts

BACKGROUND

The connection between testosterone and territoriality in free-living songbirds has been well studied in a reproductive context, but less so outside the breeding season. To assess the effects of seasonal androgenic action on territorial behavior, we analyzed vocal and non-vocal territorial behavior in response to simulated territorial intrusions (STIs) during three life-cycle stages in free-living male black redstarts: breeding, molt and nonbreeding. Concurrently, we measured changes in circulating testosterone levels, as well as the mRNA expression of androgen and estrogen receptors and aromatase in the preoptic, hypothalamic and song control brain areas that are associated with social and vocal behaviors.

RESULTS

Territorial behavior and estrogen receptor expression in hypothalamic areas did not differ between stages. But plasma testosterone was higher during breeding than during the other stages, similar to androgen receptor and aromatase expression in the preoptic area. The expression of androgen receptors in the song control nucleus HVC was lower during molt when birds do not sing or sing rarely, but similar between the breeding and the nonbreeding stage. Nevertheless, some song spectral features and the song repertoire differed between breeding and nonbreeding. Territorial behavior and song rate correlated with the expression of steroid receptors in hypothalamic areas, and in the song control nucleus lMAN.

CONCLUSIONS

Our results demonstrate seasonal modulation of song, circulating testosterone levels, and brain sensitivity to androgens, but a year-round persistency of territorial behavior and estrogen receptor expression in all life-cycle stages. This suggests that seasonal variations in circulating testosterone concentrations and brain sensitivity to androgens is widely uncoupled from territorial behavior and song activity but might still affect song pattern. Our study contributes to the understanding of the complex comparative neuroendocrinology of song birds in the wild.

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.

Neurotensin and neurotensin receptor 1 mRNA expression in song-control regions changes during development in male zebra finches

Learned vocalizations are important for communication in some vertebrate taxa. The neural circuitry for the learning and production of vocalizations is well known in songbirds, many of which learn songs initially during a critical period early in life. Dopamine is essential for motor learning, including song learning, and dopamine-related measures change throughout development in song-control regions such as HVC, the lateral magnocellular nucleus of the anterior nidopallium (LMAN), Area X, and the robust nucleus of the arcopallium (RA). In mammals, the neuropeptide neurotensin strongly interacts with dopamine signaling. This study investigated a potential role for the neurotensin system in song learning by examining how neurotensin (Nts) and neurotensin receptor 1 (Ntsr1) expression change throughout development. Nts and Ntsr1 mRNA expression was analyzed in song-control regions of male zebra finches in four stages of the song learning process: pre-subsong (25 days posthatch; dph), subsong (45 dph), plastic song (60 dph), and crystallized song (130 dph). Nts expression in LMAN during the subsong stage was lower compared to other time points. Ntsr1 expression was highest in HVC, Area X, and RA during the pre-subsong stage. Opposite and complementary expression patterns for the two genes in song nuclei and across the whole brain suggest distinct roles for regions that produce and receive Nts. The expression changes at crucial time points for song development are similar to changes observed in dopamine studies and suggest Nts may be involved in the process of vocal learning. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 78: 671-686, 2018.

Androgen and estrogen sensitivity of bird song: a comparative view on gene regulatory levels

Singing of songbirds is sensitive to testosterone and its androgenic and estrogenic metabolites in a species-specific way. The hormonal effects on song pattern are likely mediated by androgen receptors (AR) and estrogen receptor alpha (ERα), ligand activated transcription factors that are expressed in neurons of various areas of the songbirds' vocal control circuit. The distribution of AR in this circuit is rather similar between species while that of ERα is species variant and concerns a key vocal control area, the HVC (proper name). We discuss the regulation of the expression of the cognate AR and ERα and putative splice variants. In particular, we suggest that transcription factor binding sites in the promoter of these receptors differ between bird species. Further, we suggest that AR- and ERα-dependent gene regulation in vocal areas differs between species due to species-specific DNA binding sites of putative target genes that are required for the transcriptional activity of the receptors. We suggest that species differences in the distribution of AR and ERα in vocal areas and in the genomic sensitivity to these receptors contribute to species-specific hormonal regulation of the song.

Expression of sex hormone receptors in the brain of male and female newly hatched chicks

Chromosomal sex and steroid hormones play a determining role in brain sexual differentiation during chick embryonic development. Hormone effects on the brain are associated with the expression pattern of their intracellular receptors, which is sexually dimorphic in many species. We determined by Western blot the content of progesterone, estrogen, and androgen receptors (PR-A and PR-B, ERα, and AR, respectively) in the cortex, cerebellum, tectum, and hypothalamus of female and male newly hatched chicks. Males presented a higher content of PR-B in the tectum whereas females exhibited a higher content of PR-A in the hypothalamus. ERα was only detected as a band of 66kDa, and it showed a higher content in the cerebellum and tectum of females as compared to these regions in males. Besides, males exhibited a higher content of AR in the tectum than females. Our study suggests that newly hatched chicks show a sexual dimorphism in the expression of sex hormone receptors in brain regions involved in sexual behavior such as the hypothalamus, and in non-sexual behavior such as the optic tectum and the cerebellum.

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

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

Differential androgen receptor expression and DNA methylation state in striatum song nucleus Area X between wild and domesticated songbird strains

In songbirds, a specialized neural system, the song system, is responsible for acquisition and expression of species-specific vocal patterns. We report evidence for differential gene expression between wild and domesticated strains having different learned vocal phenotypes. A domesticated strain of the wild white-rumped munia, the Bengalese finch, has a distinct song pattern with a more complicated syntax than the wild strain. We identified differential androgen receptor (AR) expression in basal ganglia nucleus Area X GABAergic neurons between the two strains, and within different domesticated populations. Differences in AR expression were correlated with the mean coefficient of variation of the inter-syllable duration in the two strains. Differential AR expression in Area X was observed before the initiation of singing, suggesting that inherited and/or early developmental mechanisms may affect expression within and between strains. However, there were no distinct differences in regions upstream of the AR start codon among all the birds in the study. In contrast, an epigenetic modification, DNA methylation state in regions upstream of AR in Area X, was observed to differ between strains and within domesticated populations. These results provide insight into the molecular basis of behavioral evolution through the regulation of hormone-related genes and demonstrate the potential association between epigenetic modifications and behavioral phenotype regulation.

Testosterone modulation of angiogenesis and neurogenesis in the adult songbird brain

Throughout life, new neurons arise from the ventricular zone of the adult songbird brain and are recruited to the song control nucleus higher vocal center (HVC), from which they extend projections to its target, nucleus robustus of the arcopallium (RA). This process of ongoing parenchymal neuronal addition and circuit integration is both triggered and modulated by seasonal surges in systemic testosterone. Brain aromatase converts circulating testosterone to estradiol, so that HVC is concurrently exposed to both androgenic and estrogenic stimulation. These two signals cooperate to trigger HVC endothelial cell division and angiogenesis, by inducing the regionally-restricted expression of vascular endothelial growth factor (VEGF), its matrix-releasing protease MMP9, and its endothelial receptor VEGFR2. The expanded HVC microvascular network then secretes the neurotrophic factor BDNF, which in turn supports the recruitment of newly generated neurons. This process is striking for its spatial restriction and hence functional specificity. While androgen receptors are broadly expressed by the nuclei of the vocal control system, estrogen receptor (ERα) expression is largely restricted to HVC and its adjacent mediocaudal neopallium. The geographic overlap of these receptor phenotypes in HVC provides the basis for a regionally-defined set of paracrine interactions between the vascular bed and neuronal progenitor pool that both characterize and distinguish this nucleus. These interactions culminate in the focal attraction of new neurons to the adult HVC, the integration of those neurons into the extant vocal control circuits, and ultimately the acquisition and elaboration of song.

Examining a pathway for hormone mediated maternal effects--yolk testosterone affects androgen receptor expression and endogenous testosterone production in young chicks (Gallus gallus domesticus)

In vertebrates maternal androgens can substantially influence developing offspring, inducing both short and long term changes in physiology and behavior, including androgen sensitive traits. However, how the effects of maternal hormones are mediated remains unknown. Two possible pathways are that maternal androgens affect parts of the hypothalamus-pituitary-gonadal axis (HPG axis) or the sensitivity to androgens by affecting androgen receptor (AR) densities within the brain. To investigate both pathways, testosterone within the physiological range or vehicle only was injected into the egg yolk of unincubated chicken eggs and AR mRNA expression in different brain nuclei as well as plasma testosterone levels were measured in two week old male and female chicks that had hatched from these eggs. Our results showed a significant sex difference in plasma testosterone levels with males showing higher levels than females. Furthermore, AR mRNA expression as well as plasma testosterone levels were significantly lower in chicks hatched from testosterone treated eggs. These results suggest a compensatory mechanism for avoiding potential detrimental effects of high testosterone levels.

Distribution of androgen receptor mRNA expression in vocal, auditory, and neuroendocrine circuits in a teleost fish

Across all major vertebrate groups, androgen receptors (ARs) have been identified in neural circuits that shape reproductive-related behaviors, including vocalization. The vocal control network of teleost fishes presents an archetypal example of how a vertebrate nervous system produces social, context-dependent sounds. We cloned a partial cDNA of AR that was used to generate specific probes to localize AR expression throughout the central nervous system of the vocal plainfin midshipman fish (Porichthys notatus). In the forebrain, AR mRNA is abundant in proposed homologs of the mammalian striatum and amygdala, and in anterior and posterior parvocellular and magnocellular nuclei of the preoptic area, nucleus preglomerulosus, and posterior, ventral and anterior tuberal nuclei of the hypothalamus. Many of these nuclei are part of the known vocal and auditory circuitry in midshipman. The midbrain periaqueductal gray, an essential link between forebrain and hindbrain vocal circuitry, and the lateral line recipient nucleus medialis in the rostral hindbrain also express abundant AR mRNA. In the caudal hindbrain-spinal vocal circuit, high AR mRNA is found in the vocal prepacemaker nucleus and along the dorsal periphery of the vocal motor nucleus congruent with the known pattern of expression of aromatase-containing glial cells. Additionally, abundant AR mRNA expression is shown for the first time in the inner ear of a vertebrate. The distribution of AR mRNA strongly supports the role of androgens as modulators of behaviorally defined vocal, auditory, and neuroendocrine circuits in teleost fish and vertebrates in general.

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.

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.

Seasonal-like growth and regression of the avian song control system: neural and behavioral plasticity in adult male Gambel's white-crowned sparrows

Birdsong is regulated by a series of discrete brain nuclei known as the song control system. In seasonally-breeding male songbirds, seasonal changes in steroid sex hormones regulate the structure and electrophysiology of song control system neurons, resulting in dramatic changes in singing behavior. Male songbirds can be brought into the laboratory, where circulating levels of steroid hormone and photoperiod can be abruptly manipulated, providing controlled conditions under which rapid "seasonal-like" changes in behavior and morphology can be carefully studied. In this mini-review, we discuss the steroidal and cellular mechanisms underlying seasonal-like growth and regression of the song control system in adult male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), and its impact on song behavior. Specifically, we discuss recent advances concerning: (1) the role of androgen and estrogen receptors in inducing seasonal-like growth of the song control system; (2) how photoperiod modulates the time course of testosterone-induced growth of the song control system; (3) how bilateral intracerebral infusion of androgen and estrogen receptor antagonists near the song control nucleus HVC prevents seasonal-like increases in song stereotypy but not song rate; and (4) the steroidal and cellular mechanisms that mediate rapid regression of the song control system. Throughout this mini-review we compare data collected from white-crowned sparrows to that from other songbird species. We conclude by outlining avenues of future research.

Socially induced brain differentiation in a cooperatively breeding songbird

Birds living in social groups establish dominance hierarchies, and taking up the dominant position influences behaviour and physiological parameters. In cooperatively breeding white-browed sparrow weavers (Plocepasser mahali), the transition from subordinate helper to dominant breeder male induces the production of a new type of song. This song contains a large number of new syllables and differs in temporal pattern from duet songs produced by all other group members. Here we show that this change in social status of adult males affects the morphology of a behavioural control circuit, the song control system of songbirds that is composed of large neuron populations. The volume of the song control areas HVC and RA and their gene-expression levels depend on males' social status. Dominant males have several times larger testes than subordinates, which is not reflected in circulating androgen and oestrogen levels. Our findings suggest a remarkable differentiation of adult vertebrate brains in relation to changing social cues.

Steroid hormones act transsynaptically within the forebrain to regulate neuronal phenotype and song stereotypy

Steroid sex hormones induce dramatic seasonal changes in reproductive related behaviors and their underlying neural substrates in seasonally breeding vertebrates. For example, in adult white-crowned sparrows, increased Spring photoperiod raises circulating testosterone, causing morphological and electrophysiological changes in song-control nuclei, which modify song behavior for the breeding season. We investigated how photoperiod and steroid hormones induce these changes in morphology, electrophysiology, and behavior. Neurons in a song premotor nucleus, the robust nucleus of the arcopallium (RA), show increased intrinsic spontaneous firing rate and soma size when birds are in breeding condition. Using combinations of systemic and unilateral local intracerebral hormonal manipulations, we show that long-day photoperiod accelerates the effects of systemic testosterone on RA neurons via the estradiol-synthesizing enzyme aromatase (CYP19A1); these changes require inputs from the afferent song control nucleus HVC (used as a proper name) and steroid receptor activation within HVC; local coactivation of androgen and estrogen receptors (ARs and ERs, respectively) within HVC, but not RA, is sufficient to cause neuronal changes in RA; activation of ARs in RA is also permissive. Using bilateral local intracerebral hormone-receptor blockade, we found that ARs and ERs in the song-control nucleus HVC mediate systemic testosterone-induced changes in song stereotypy but not rate. This novel transsynaptic effect of gonadal steroids on activity and morphology of RA neurons is part of a concerted change in key premotor nuclei, enabling stereotyped song.

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

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

Enriched expression and developmental regulation of the middle-weight neurofilament (NF-M) gene in song control nuclei of the zebra finch

Songbirds evolved a complex set of dimorphic telencephalic nuclei that are essential for the learning and production of song. These nuclei, which together make up the oscine song control system, present several neurochemical properties that distinguish them from the rest of the telencephalon. Here we show that the expression of the gene encoding the middle-weight neurofilament (NF-M), an important component of the neuronal cytoskeleton and a useful tool for studying the cytarchitectonic organization of mammalian cortical areas, is highly enriched in large neurons within pallial song control nuclei (nucleus HVC, robustus nucleus of the arcopallium, and lateral magnocellular nucleus of the nidopallium) of male zebra finches (Taeniopygia guttata). We also show that this transcript is highly expressed in large neurons in the medulla, pons, midbrain, and thalamus. Moreover, we demonstrate that NF-M expression in song control nuclei changes during postembryonic development, peaking during an early phase of the song-learning period that coincides with the maturation of the song system. We did not observe changes in NF-M expression in auditory areas or in song control nuclei in the contexts of hearing song or singing, although these contexts result in marked induction of the transcription factor ZENK. This observation suggests that NF-M might not be under the regulatory control of ZENK in auditory areas or in song control nuclei. Overall, our data indicate that NF-M is a neurochemical marker for pallial song control nuclei and provide suggestive evidence of an involvement of NF-M in the development and/or maturation of the oscine song control system.

Sexual Differentiation of the Vocal Control System of Birds

Birds evolved neural circuits of various complexities in relation to their capacity to produce learned or unlearned vocalizations. These vocalizations, in particular those that function in the realm of reproduction, are frequently sexually dimorphic, both in vocal learners (songbirds, parrots, some hummingbirds) and vocal nonlearners (all other birds). In many cases, the development and/or the adult differentiation of vocalizations of sociosexual function is sensitive to sex hormones, androgens and estrogens. The underlying mechanisms have been studied in detail in songbirds, a bird group that comprises about half of all bird species. Next to unlearned calls, songbirds produce learned songs that require forebrain vocal control areas that express receptors for androgens and estrogens. These forebrain vocal areas are sexually dimorphic in many species, but a clear relation between the degree of "brain sex" and sex differences in vocal pattern is lacking, except that a minimum number of vocal neurons is necessary to sing learned songs. Genetic brain-intrinsic mechanisms are likely to determine the neuron pools that develop into forebrain song control areas. Subsequently, gonadal steroid hormones, androgens and estrogens, modulate the fate of these neurons and thus the functionality of the vocal control systems. Further action of gonadal hormones, and may be other factors signaling the sociosexual and physical environment, affect the phenotype of vocal control areas in adulthood. Despite the clear evidence of hormone dependency of both adult vocalizations and phenotypes of vocal neuron pools, their causal relation is little understood.

Sex-role reversal is reflected in the brain of African black coucals (Centropus grillii)

In most bird species males compete over access to females and have elevated circulating androgen levels when they establish and defend a breeding territory or guard a mate. Testosterone is involved in the regulation of territorial aggression and sexual display in males. In few bird species the traditional sex-roles are reversed and females are highly aggressive and compete over access to males. Such species represent excellent models to study the hormonal modulation of aggressive behavior in females. Plasma sex steroid concentrations in sex-role reversed species follow the patterns of birds with "traditional" sex-roles. The neural mechanisms modulating endocrine secretion and hormone-behavior interactions in sex-role reversed birds are currently unknown. We investigated the sex differences in the mRNA expression of androgen receptors, estrogen receptor alpha, and aromatase in two brain nuclei involved in reproductive and aggressive behavior in the black coucal, the nucleus taeniae and the bed nucleus of the stria terminalis. In the bed nucleus there were no sex differences in the receptor or aromatase expression. In the nucleus taeniae, however, we show for the first time, that females have a higher mRNA expression of androgen receptors than males. These results suggest that the expression of agonistic and courtship behavior in females does not depend on elevated blood hormone levels, but may be regulated via increased steroid hormone sensitivity in particular target areas in the brain. Hence, aggression in females and males may indeed be modulated by the same hormones, but regulated at different levels of the neuroendocrine cascade.

Utilization of a zebra finch BAC library to determine the structure of an avian androgen receptor genomic region

The zebra finch (Taeniopygia guttata) is an important model organism for studying behavior, neuroscience, avian biology, and evolution. To support the study of its genome, we constructed a BAC library (TG__Ba) using DNA from livers of females. The BAC library consists of 147,456 clones with 98% containing inserts of an average size of 134 kb and represents 15.5 haploid genome equivalents. By sequencing a whole BAC, a full-length androgen receptor open reading frame was identified, the first in an avian species. Comparison of BAC end sequences and the whole BAC sequence with the chicken genome draft sequence showed a high degree of conserved synteny between the zebra finch and the chicken genome.

Hormonal influence on song structure and organization: The role of estrogen

The development of song in songbirds is a complex phenomenon that involves memory and learning, sensorimotor integration, and neural and muscular maturation. Gonadal hormones are involved in each of these steps, as they influence the differentiation of the neural song system, the incorporation and survival of neurones, and the development of muscles used for song production. In young males the development of song, therefore, is closely linked to the secretion of testosterone by the testicles. Castration results in the development of incomplete or unstable songs, and hormone replacement leads to the development of crystallized or stable song. However, testosterone does not act solely as an androgen. The brain of songbirds contains high concentrations of the enzyme aromatase, which converts testosterone into estradiol. Estradiol then binds to estrogen receptors, which in the song system are found only in the nucleus HVC. This forebrain nucleus, also called the "master nucleus," codes for the syntactic structure of song, i.e. for the particular combination of simple elements-syllables-that characterize the song of an individual. In this paper, we will review our studies on the role of estrogen in guiding the organization of song in canaries.

Seasonal-like plasticity of spontaneous firing rate in a songbird pre-motor nucleus

Many animals exhibit seasonal changes in behavior and its underlying neural substrates. In seasonally breeding songbirds, the brain nuclei that control song learning and production undergo substantial structural changes at the onset of each breeding season, in association with changes in song behavior. These changes are largely mediated by photoperiod-dependent changes in circulating concentrations of gonadal steroid hormones. Little is known, however, about whether changes in the electrophysiological activity of neurons accompany the dramatic morphological changes in the song nuclei. Here we induced seasonal-like changes in the song systems of adult white-crowned sparrows and used extracellular recording in acute brain slices from those individuals to study physiological properties of neurons in the robust nucleus of the arcopallium (RA), a pre-motor nucleus necessary for song production. We report that: RA neurons from birds in breeding condition show a more than twofold increase in spontaneous firing rate compared to those from nonbreeding condition; this change appears to require both androgenic and estrogenic actions; and this change is intrinsic to the RA neurons. Thus, neurons in the song circuit exhibit both morphological and physiological adult seasonal plasticity.

Modulation by steroid hormones of a ''sexy'' acoustic signal in an Oscine species, the Common Canary Serinus canaria

The respective influence of testosterone and estradiol on the structure of the Common Canary Serinus canaria song was studied by experimentally controlling blood levels of steroid hormones in males and analyzing the consequent effects on acoustic parameters. A detailed acoustic analysis of the songs produced before and after hormonal manipulation revealed that testosterone and estradiol seem to control distinct song parameters independently. The presence of receptors for testosterone and estradiol in the brain neural pathway controlling song production strongly suggests that the observed effects are mediated by a steroid action at the neuronal level.

Songbird Genomics: Methods, Mechanisms, Opportunities, and Pitfalls

The biology of songbirds poses fundamental questions about the interplay between gene, brain, and behavior. New tools of genomic analysis will be invaluable in pursuing answers to these questions. This review begins with a summary of the broad properties of the songbird genome and how songbird brain gene expression has been measured in past studies. Four key problems in songbird biology are then considered from a genomics perspective: What role does differential gene expression play in the development, maintenance, and functional organization of the song control circuit? Does gene regulation set boundaries on the process of juvenile song learning? What is the purpose of song-induced gene activity in the adult brain? How does the genome underlie the profound sexual differentiation of the song control circuit? Finally, the range of genomic technologies currently or soon to be available to songbird researchers is briefly reviewed. These technologies include online databases of expressed genes ("expressed sequence tags" or ESTs); a complete library of the zebra finch genome maintained as a bacterial artificial chromosome (BAC) library; DNA microarrays for simultaneous measurement of many genes in a single experiment; and techniques for gene manipulation in the organism. Collectively, these questions and techniques define the field of songbird neurogenomics.

Hormone-Dependent Neural Plasticity in the Juvenile and Adult Song System: What Makes a Successful Male?

The sexual quality of adult song is the result of genetic and epigenetic mechanisms shaping the neural song system throughout life. Genetic brain-intrinsic mechanisms determine the neuron pools that develop into forebrain song control areas independent of gonadal steroid hormones, androgens and estrogens. One fate of these neurons is the potential to express sex steroid receptors, such as androgen and estrogen receptors. Genetic brain-intrinsic mechanisms, too, determine the activity of hypothalamic-pituitary-gonad (HPG) axis, i.e., the working range and responsiveness of HPG axis to produce gonadal hormones. The epigenetic action of gonadal steroid hormones (androgens and estrogens) on determined vocal neurons is required to maintain and increase the pool of determined vocal neurons and to complete the connections of the vocal system, i.e., to make it function motorically. The subsequent influence of environmental information, including both external (socio-sexual and physical) and internal (body physiology) signals, specify the further neural phenotype of vocal areas either through acting on the HPG axis and differential release of gonadal hormones or through non-gonadal hormone systems, both of which have target neurons in the functional vocal system. Despite the clear evidence of hormone dependency of the development of both the adult song phenotype and song system phenotype, their causal relation is complex.

The activation of birdsong by testosterone: multiple sites of action and role of ascending catecholamine projections

Birdsong is a species-typical stereotypic vocalization produced in the context of reproduction and aggression. Among temperate-zone songbirds, it is produced primarily by males, and its frequency and quality are enhanced by the presence of the gonadal steroid hormone testosterone in the plasma. In the brain, the effects of testosterone on song behavior involve both estrogenic and androgenic metabolites of testosterone that are locally produced and act via their cognate receptors. Androgen, and in some cases estrogen, receptors are present in many specialized forebrain song control nuclei. Testosterone can regulate catecholamine steady-state levels and turnover in these song control regions. Tracing studies combined with immunocytochemistry for tyrosine hydroxylase (a marker of catecholamine synthesis) reveal several catecholamine cell groups that project to forebrain song control nuclei. These brain areas also express the mRNA for either androgen receptors or estrogen receptor alpha, and androgens enhance the expression of tyrosine hydroxylase. Dopaminergic cell groups that project to song nuclei express the protein product of the immediate early gene fos in association with the production of territorial song. Thus, testosterone may be acting on song behavior via these ascending catecholamine cell groups. Chemical lesioning studies suggest that noradrenergic projections to the song system are involved in the latency to produce song and the ability to discriminate conspecific from heterospecific song. The song control circuit may thus be modulated in significant ways via the androgen regulation of forebrain catecholamine systems.

Song activation by testosterone is associated with an increased catecholaminergic innervation of the song control system in female canaries

In canaries, singing and a large number of morphological features of the neural system that mediates the learning, perception and production of song exhibit marked sex differences. Although these differences have been mainly attributed to sex-specific patterns of the action of testosterone and its metabolites, the mechanisms by which sex steroids regulate brain and behavior are far from being completely understood. Given that the density of immunoreactive catecholaminergic fibers that innervate telencephalic song nuclei in canaries is higher in males, which sing, than in females, which usually do not sing, we hypothesized that some of the effects induced by testosterone on song behavior are mediated through the action of the steroid on the catecholaminergic neurons which innervate the song control nuclei. Therefore, we investigated in female canaries the effects of a treatment with exogenous testosterone on song production, on the volume of song control nuclei, and on the catecholaminergic innervation of these nuclei as assessed by immunocytochemical visualization of tyrosine hydroxylase. Testosterone induced male-like singing in all females and increased by about 80% the volume of two telencephalic song control nuclei, the high vocal center (HVC) and the nucleus robustus archistriatalis (RA). Testosterone also significantly increased the fractional area covered by tyrosine hydroxylase-immunoreactive structures (fibers and varicosities) in most telencephalic song control nuclei (HVC, the lateral and medial parts of the magnocellular nucleus of the anterior neostriatum, the nucleus interfacialis, and to a lesser extent RA). By contrast, testosterone did not affect the catecholaminergic innervation of the telencephalic areas adjacent to HVC and RA. Together these data demonstrate that, in parallel to its effects on song behavior and on the morphology of the song control system, testosterone also regulates the catecholaminergic innervation of most telencephalic song control nuclei in canaries. The endocrine regulation of singing may thus involve the neuromodulatory action of specialized dopaminergic and/or noradrenergic projections onto several key parts of the song control system.

Expression of androgen receptor mRNA in zebra finch song system: Developmental regulation by estrogen

By using improved methods for in situ hybridization to detect expression of androgen receptor (AR) mRNA, the distribution of expression was mapped in the adult male zebra finch brain. In the neural song circuit, robust expression was found in area X of the lobus parolfactorius (LPO) as well as in other song regions previously reported. Expression was also found in many areas of the hypothalamus and dorsal thalamic nuclei, nucleus intercollicularis and ventricular areas of the midbrain, cerebellar Purkinje and granule cells, the hyperstriatum, medial neostriatum, medial LPO, and archistriatum. In juvenile males, AR mRNA expression was first detected in nucleus high vocal center (HVC) at posthatch day 9 (P9), in area X at P9-P11, and in the region of the robust nucleus (RA) in the medial archistriatum by P7. Estrogen treatment of hatchling females caused an increase in the expression of AR mRNA in HVC and area X by P11, whereas treatment of hatchling males with the aromatase inhibitor fadrozole decreased the expression of AR mRNA at P11. The present results indicate that masculine development of AR expression begins in area X and HVC before they are thought to be synaptically connected, suggesting that different song nuclei initiate sexual differentiation independently of transsynaptic masculinizing influences. The present results suggest that estrogen is necessary for full masculine AR expression in the song system and that the estrogenic regulation of AR contributes to subsequent differential actions of androgen in male and female song nuclei.

Aromatase inhibition affects testosterone-induced masculinization of song and the neural song system in female canaries

Songbirds have a specialized steroid-sensitive network of brain nuclei, the song system, for controlling song. Most nuclei of the song system express androgen receptors, and the sensory-motor integration nucleus High Vocal Center (HVC) alone also expresses estrogen receptors. Apart from expressing estrogen receptors in the vocal control system, songbirds are unique among birds because they have high concentrations of the estrogen-synthesizing enzyme aromatase in the neostriatum surrounding HVC. However, the role of estrogen in controlling the development of the song structure has been scarcely investigated. In this work, we show that blocking the production of estrogen during testosterone-induced song motor development in adult female canaries alters the song pattern compared to control females treated with testosterone only. These effects were correlated with inhibition of the expression of estrogen-sensitive genes, such as brain-derived nerve growth factor, in HVC. The expression of the ATP-synthase gene, an indicator of cell activity, in HVC, and the size of HVC, were not affected by the treatment. Our results provide the first example of estrogen-sensitive mechanisms controlling the structural features of adult birdsong.

Sex differences in the distribution of the steroid receptor coactivator SRC-1 in the song control nuclei of male and female canaries

The steroid receptor coactivator SRC-1 modulates ligand-dependent transactivation of several nuclear receptors, including the receptors for sex steroid hormones. The distribution of SRC-1 transcripts was analyzed here by in situ hybridization in coronal sections through the brain of male and female canaries. A broad but heterogeneous distribution of SRC-1 transcripts was observed with high numbers of densely labeled cells being present in many steroid-sensitive areas including the medial preoptic nucleus, several hypothalamic nuclei, five song control nuclei (HVc, the lateral and medial portion of the magnocellular nucleus of the anterior neostriatum, area X and the nucleus uvaeformis) and several catecholaminergic areas (area ventralis of Tsai, substantia nigra, locus coeruleus). The volume of two song control nuclei, HVc and area X were reconstructed based on the boundaries of the cell groups exhibiting a denser SRC-1 expression as compared to the surrounding areas. Sex differences in the expression of SRC-1 were also detected in several song control nuclei. In particular, the volume of HVc based on the high density of SRC-1 expression was significantly larger in males than in females. The effect of steroids on the song control system could be, at least in part, indirect and result from a modulation by steroids of the catecholaminergic inputs to the song control nuclei. The presence of the steroid receptor coactivator SRC-1 in the telencephalic song control nuclei and in the catecholaminergic cell groups that innervate the song system supports the idea that SRC-1 expression could play an active role in the control of singing behavior by modulating estrogen and androgen receptor action at both locations.

Coordinated interaction of neurogenesis and angiogenesis in the adult songbird brain

Neurogenesis proceeds throughout life in the higher vocal center (HVC) of the adult songbird neostriatum. Testosterone induces neuronal addition and endothelial division in HVC. We asked if testosterone-induced angiogenesis might contribute importantly to HVC neuronal recruitment. Testosterone upregulated both VEGF and its endothelial receptor, VEGF-R2/Quek1/KDR, in HVC. This yielded a burst in local HVC angiogenesis. FACS-isolated HVC endothelial cells produced BDNF in a testosterone-dependent manner. In vivo, HVC BDNF rose by the third week after testosterone, lagging by over a week the rise in VEGF and VEGF-R2. In situ hybridization revealed that much of this induced BDNF mRNA was endothelial. In vivo, both angiogenesis and neuronal addition to HVC were substantially diminished by inhibition of VEGF-R2 tyrosine kinase. These findings suggest a causal interaction between testosterone-induced angiogenesis and neurogenesis in the adult forebrain.

Neuroendocrinology of song behavior and avian brain plasticity: multiple sites of action of sex steroid hormones

Seasonal changes in the brain of songbirds are one of the most dramatic examples of naturally occurring neuroplasticity that have been described in any vertebrate species. In males of temperate-zone songbird species, the volumes of several telencephalic nuclei that control song behavior are significantly larger in the spring than in the fall. These increases in volume are correlated with high rates of singing and high concentrations of testosterone in the plasma. Several song nuclei express either androgen receptors or estrogen receptors, therefore it is possible that testosterone acting via estrogenic or androgenic metabolites regulates song behavior by seasonally modulating the morphology of these song control nuclei. However, the causal links among these variables have not been established. Dissociations among high concentrations of testosterone, enlarged song nuclei, and high rates of singing behavior have been observed. Singing behavior itself can promote cellular changes associated with increases in the volume of the song control nuclei. Also, testosterone may stimulate song behavior by acting in brain regions outside of the song control system such as in the preoptic area or in catecholamine cell groups in the brainstem. Thus testosterone effects on neuroplasticity in the song system may be indirect in that behavioral activity stimulated by testosterone acting in sites that promote male sexual behavior could in turn promote morphological changes in the song system.

Natural breeding conditions and artificial increases in testosterone have opposite effects on the brains of adult male songbirds: a meta-analysis

A meta-analysis of the literature shows that in adult male songbirds, brain mass, telencephalon volume and n. rotundus (a thalamic visual nucleus) volume increase from the nonbreeding season (low testosterone) to the breeding season (higher testosterone). These effects can at least partially be mimicked by photoperiod manipulations in captivity. In contrast, an artificial testosterone (T) titer increase by chronic implants yields the opposite results: telencephalon, n. rotundus, and n. pretectalis volumes are lower in T-treated animals than in controls. These results suggest that artificial testosterone manipulations do not necessarily mimic the effects of natural variations in hormone levels and that results from experiments using T implants to mimic natural hormonal effects should be interpreted with caution.

Androgen receptor immunolocalization in brains of courting and brooding male and female ring doves (Streptopelia risoria)

Nuclear androgen receptors (ARs) were localized immunocytochemically in the brains of courting and brooding male and female ring doves (Streptopelia risoria). AR immunoreactivity (AR-ir) in courting birds was localized in cell nuclei in the telencephalon, diencephalon, and mesencephalon. In the anterior hypothalamus, high density of AR-ir was concentrated in several nuclei including the nucleus lateralis hypothalami, nucleus periventricularis magnocellularis, nucleus preopticus anterior, nucleus preopticus medialis, and nucleus preopticus paraventricularis magnocellularis. In the posterior hypothalamus, areas showing high density of AR-ir included the nucleus lateralis hypothalami posterioris, nucleus medialis hypothalami posterior, nucleus ectomamillaris, nucleus mamillaris lateralis, and nucleus tuberis. No sex differences in the density or localization of AR-ir were observed. Compared to brains from courting birds, AR-ir density was either extremely low or absent in most brain regions of brooding birds. It is concluded that in the dove, central ARs are closely associated with the sexual stages of the reproductive cycle.

Gonadal steroid receptor mRNA in catecholaminergic nuclei of the canary brainstem

Steroid actions in the song system may be modulated by ascending inputs from catecholaminergic (CA) brain nuclei; however, whether these nuclei contain steroid receptors is unknown. Here, we compared the distribution of androgen receptor (AR) and estrogen receptor-alpha (ER-alpha) mRNA with that of tyrosine hydroxylase immunoreactivity (TH-IR) in the brainstems of male canaries. Areas containing AR and ER-alpha mRNA overlapped with areas containing TH-IR cell bodies in the locus ceruleus and the area ventralis of Tsai. The substantia nigra and the midbrain central gray contained both TH-IR and AR mRNA. The presence of AR and ER-alpha within CA cell groups suggests that sex steroid hormones may modulate song production at the site of CA synthesis.

Distribution of sex steroid hormone receptors in the avian brain: functional implications for neural sex differences and sexual behaviors

Developmental and seasonal changes in the production of androgens, estrogens, and progestins seem to control sex-specific differentiation and seasonal changes in appetitive and consummatory sexual behaviors of birds. This results in profound sex differences in the quality (sex-specific) or quantity (sex-typical) of behaviors such as courtship, territoriality, or copulation. Steroids affect the brain by binding to intracellularly located receptors. The same brain areas express androgen, estrogen, and progesterone receptors in male and female brains. Sex differences in these genetically determined patterns occur in the size of neuron populations that intrinsically express sex steroid receptors. Further permanent sex differences are subsequent to degenerative fates of receptor expressing neuron populations during ontogeny. Transient sex differences in receptor expression appear to be due to area-specific up- and down-regulation of receptor levels, reflecting transient changes in the level of circulating steroids, changes in environmental conditions, or in the physiological status of the individuals. In particular, intrinsic sex differences in the expression pattern of sex steroid receptors and steroid-independent regulation of the expression level of these receptors in the brain are limiting mechanisms for gonad-dependent sexual development and activities.

Androgen receptor, estrogen receptor alpha, and estrogen receptor beta show distinct patterns of expression in forebrain song control nuclei of European starlings

In songbirds, singing behavior is controlled by a discrete network of interconnected brain nuclei known collectively as the song control system. Both the development of this system and the expression of singing behavior in adulthood are strongly influenced by sex steroid hormones. Although both androgenic and estrogenic steroids have effects, androgen receptors (AR) are more abundantly and widely expressed in song nuclei than are estrogen receptors (ER alpha). The recent cloning of a second form of the estrogen receptor in mammals, ER beta, raises the possibility that a second receptor subtype is present in songbirds and that estrogenic effects in the song system may be mediated via ER beta. We therefore cloned the ER beta complementary DNA (cDNA) from a European starling preoptic area-hypothalamic cDNA library and used in situ hybridization histochemistry to examine its expression in forebrain song nuclei, relative to the expression of AR and ER alpha messenger RNA (mRNA), in the adjacent brain sections. The starling ER beta cDNA has an open reading frame of 1662-bp, predicted to encode a protein of 554 amino acids. This protein shares greater than 70% sequence identity with ER beta in other species. We report that starling ER beta is expressed in a variety of tissues, including brain, pituitary, skeletal muscle, liver, adrenal, kidney, intestine, and ovary. Similar to reports in other songbird species, we detected AR mRNA-containing cells in several song control nuclei, including the high vocal center (HVc), the medial and lateral portions of the magnocellular nucleus of the anterior neostriatum, and the robust nucleus of the archistriatum. We detected ER alpha expression in the medial portion of HVc (also called paraHVc) and along the medial border of the caudal neostriatum. ER beta was not expressed in HVc, in the medial and lateral portions of the magnocellular nucleus of the anterior neostriatum, in the robust nucleus of the archistriatum, or in area X. In contrast, ER beta mRNA-containing cells were detected in the caudomedial neostriatum and medial preoptic area in a pattern reminiscent of P450 aromatase expression in the same brain regions in other songbirds. These data suggest that estrogenic effects on the song system are not mediated via ER beta-producing cells within song nuclei. Nonetheless, the overlapping expression of ER beta- and aromatase-producing cells in the caudomedial neostriatum suggests that locally synthesized estrogens may act via ER beta, in addition to ER alpha, to mediate seasonal or developmental effects on nearby song nuclei (e.g. HVc).

Seasonal changes in androgen receptor immunoreactivity in the song nucleus HVc of a wild bird

In seasonally breeding songbirds, song behavior and neural morphology change seasonally. Song control nuclei are larger during the breeding season, as determined by multiple cytological labels. Seasonal changes in song nuclei are regulated by testosterone (T), and several song nuclei contain intracellular androgen receptors (AR). Changes in AR levels may interact with changes in plasma T levels to regulate song nuclei morphology. We measured seasonal changes in AR-immunoreactive cells in the telencephalic song nucleus HVc using the affinity-purified PG21 antibody to rat AR. We caught wild adult male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) during spring breeding in Alaska and during autumn migration in Washington State. To enhance PG21 labeling, animals were treated with T for 90 minutes (as in Smith et al. [1996] J. Histochem. Cytochem. 44:1075-1080). AR+ cells were found in HVc and other song nuclei, hippocampus, nucleus taeniae (homologue to mammalian amygdala), and the hypothalamus. HVc volume was larger in spring (S) than autumn (A), in both the PG21- and Nissl-stained sections (S:A = 1.9 and 1.7, respectively). In spring, but not autumn, PG21 and Nissl measurements were slightly different (PG21:Nissl = 1.07), perhaps because PG21 labeled the most caudal extent of HVc more clearly. In HVc, AR+ cell density and number were greater in spring. The percentage of AR+ cells was also increased in spring. Qualitatively, the staining intensity of individual cells was higher in spring. In time course studies, the T injection enhanced PG21 staining within 15 minutes, suggesting that it increases labeling via AR translocation to and concentration in the cell nucleus.

Quality of life in advanced prostate cancer: results of a randomized therapeutic trial

BACKGROUND

For patients with metastatic prostate cancer, treatment is primarily palliative, relying mainly on the suppression of systemic androgen hormone levels. To help document the achievement of palliation and to characterize positive and negative effects of treatment, we evaluated quality-of-life (QOL) parameters in patients with metastatic prostate cancer who were randomly assigned to two methods of androgen deprivation.

METHODS

Patients (n = 739) with stage M1 (bone or soft tissue metastasis) prostate cancer were enrolled in a QOL protocol that was a companion to Southwest Oncology Group INT-0105, a randomized double-blind trial comparing treatment with bilateral orchiectomy (surgical castration) plus either flutamide or placebo. Patients completed a comprehensive battery of QOL questionnaires at random assignment to treatment and at 1, 3, and 6 months later. Data were collected on three treatment-specific symptoms (diarrhea, gas pain, and body image), on physical functioning, and on emotional functioning. All P values are two-sided.

RESULTS

Questionnaire return rates for this study never dropped below 80%; only 2% of the patients did not submit baseline QOL assessments. Cross-sectional analyses (corrected for multiple testing) identified statistically significant differences that favored orchiectomy plus placebo for two of the five primary QOL parameters as follows: patients receiving flutamide reported more diarrhea at 3 months (P = .001) and worse emotional functioning at 3 and 6 months (both P<.003). Longitudinal analyses replicated these findings. Other analyzed QOL parameters favored the group receiving placebo but were not statistically significant after adjustment for multiple testing.

CONCLUSIONS

We found a consistent pattern of better QOL outcomes at each follow-up assessment during the first 6 months of treatment for orchiectomized patients with metastatic prostate cancer who received placebo versus flutamide. Improvement over time was evident in both treatment groups but more so for patients receiving placebo.

Insulin-like growth factor-1 is a radial cell-associated neurotrophin that promotes neuronal recruitment from the adult songbird edpendyma/subependyma

In the adult songbird forebrain, neurons continue to be produced from precursor cells in the forebrain ependymal/subependymal zone (SZ), from which they migrate upon radial guide fibers. The new neurons and their radial cell partners may coderive from a common SZ progenitor, which may be the radial cell itself. On this basis, we asked whether radial cells might provide trophic support for the migration or survival of newly generated neurons. We focused upon the insulin-like growth factors (IGFs) IGF-1 and IGF-2, which have previously been shown to support the survival and differentiation of neural progenitor cells. We found that IGF-1 immunoreactivity was expressed heavily by adult zebra finch radial cells and their fibers, with little expression otherwise. IGF-2, in contrast, was expressed by parenchymal astrocytes and exhibited little radial cell expression. Despite their distinct distributions, IGF-1 and IGF-2 exerted similar trophic effects on finch SZ cells in vitro; both greatly increased the number of neurons migrating from explants of the adult finch SZ, relative to explants raised in low-insulin, IGF-1-deficient media. However, neither factor extended neuronal survival. These results suggest that in neurogenic regions of the adult avian forebrain, IGF-1 acts as a radial cell-associated neuronal differentiation and/or departure factor, which may serve to regulate neuronal recruitment into the adult brain.

Sex difference in the size of the neural song control regions in a dueting songbird with similar song repertoire size of males and females

Previous studies have suggested a causal relation between sex differences in behavior such as singing and sex differences in the size of brain areas such as the forebrain song control areas of songbirds. In the present study we show that the size of the forebrain vocal control areas nucleus hyperstriatalis ventrale pars caudale (HVC) and nucleus robustus archistriatalis (RA) and its neuron numbers are about twice as large in males as in females of the African dueting bush shrike Laniarius funebris. However, song types are of similar complexity (number of elements per song type, physical properties of elements) in both sexes, and repertoire size does not differ between males and females. Furthermore, in captivity male and female shrikes are able to learn the same song types. This demonstrates for the shrike that sex differences in the size of vocal control areas and in its neuron numbers do not predict the type of sex-typical vocal behavior. This result is supported by a statistical comparison of the sex differences in HVC size, RA size, and song repertoire size of all songbird species studied to date. Sex differences in species in which only the males sing are indeed larger than in species in which the females also sing; in songbird species with singing females, however, the sex differences in HVC and RA volume appear to be independent of the vocal repertoire size of females. The songbird model therefore does not support the notion that sex differences in area size and neuron number explain sex differences in a behavior that occurs in both sexes. Furthermore, in the shrike, neuron soma size is similar in males and females in the song motonucleus hypoglossus pars tracheosyringealis (nXIIts) and in the premotor nucleus RA, but is sexually dimorphic in the higher vocal center HVC. Thus, male and female shrikes produce songs of similar complexity with different neuron phenotypes.