Effects of long-term flutamide treatment during development in zebra finches

Androgen signaling in LMAN regulates song stereotypy in male canaries

During breeding when testosterone concentrations are high, male songbirds that are open-ended vocal learners like canaries (Serinus canaria) tend to produce a stable, stereotyped song that facilitates mate attraction or territory defense. Outside breeding contexts, song becomes more variable. The neuroendocrine mechanisms controlling this vocal variability across seasons are not entirely clear. We tested whether androgen signaling within the lateral magnocellular nucleus of the anterior nidopallium (LMAN), a cortical-like brain region of the vocal control system known as a vocal variability generator, plays a role in seasonal vocal variability. We first characterized song in birds housed alone on a short day (SD) photoperiod, which simulates non-breeding conditions. Then, cannulae filled with the androgen receptor (AR) blocker flutamide or left empty as control were implanted bilaterally in LMAN. Birds were then transferred to long days (LD) to simulate the breeding season and song was analyzed again. Blocking AR in LMAN increased acoustic variability of song and the acoustic variability of syllables. However, blocking AR in LMAN did not impact the variability of syllable usage nor their sequencing in LD birds, song features that are controlled by androgen signaling in a somatosensory brain region of the vocal control system called HVC. These findings highlight the multifactorial, non-redundant actions of steroid hormones in controlling complex social behaviors such as birdsong. They also support the hypothesis that LMAN is a key brain area for the effects of testosterone on song plasticity both seasonally in adults and during the song crystallization process at sexual maturity.

Life-long testosterone and antiandrogen treatments affect the survival and reproduction of captive male red-legged partridges (Alectoris rufa)

Abstract

Sexual steroids can play an important role as life-history organizers. In males, high circulating testosterone levels induce physiological/behavioral costs and benefits, leading to trade-offs. However, studies simultaneously testing the impact of these levels in both fitness components (survival and fecundity) during lifetime are scarce and limited to wild birds. To determine the mortality causes or hormonal manipulation impacts on male fertility is, nonetheless, a difficult task in free-ranging animals that could be easier in captivity. We longitudinally monitored captive red-legged partridges (Alectoris rufa) and exposed males to high exogenous testosterone levels, anti-androgens, or a control treatment during each breeding period throughout their lives. Theory predicts that individuals maintaining high androgen levels should obtain higher fitness returns via reproduction, but suffer reduced longevity. Testosterone-treated male partridges, accordingly, lived shorter compared to controls, since they were more prone to die from a natural bacterial infection. However, the same birds seemed to have a lower capacity to fertilize eggs, probably due to endocrine feedback reducing testicular mass. These results show that exogenous testosterone can exert unpredicted effects on fitness parameters. Therefore, caution must be taken when drawing conclusions from non-fully controlled experiments in the wild. Males treated with the androgen-receptor blocker flutamide did not outlive controls as predicted by the life-history trade-off theory, but their mates laid eggs with higher hatching success. The latter could be due to mechanisms improving sperm quality/quantity or influencing maternal investment in egg quality. Testosterone receptor activity/amount could thus be as relevant to fitness as testosterone levels.

Significance statement

It has repeatedly been hypothesized that high testosterone levels induce a cost in terms of reduced lifetime reproductive success. This can be due to reduced fecundity or via shorter lifespan. This is, however, only supported by a handful of studies, mostly in wild birds. We tested this in captive male red-legged partridges, which allowed us to determine reproductive success and mortality causes. We increased testosterone levels or blocked its action with antiandrogens throughout life. High testosterone levels reduced the survival by making birds more prone to die by infection. The eggs produced by their mates also showed lower hatching success, a probable manipulation artifact that should be considered in avian studies in the wild. Interestingly, the androgen-receptor blocker flutamide increased lifetime hatching success compared to controls, suggesting that androgen receptor amounts/activity are even more relevant to fitness than testosterone levels.

Female zebra finches do not sing yet share neural pathways necessary for singing in males

Adult female zebra finches (Taeniopygia guttata), which do not produce learned songs, have long been thought to possess only vestiges of the forebrain network that supports learned song in males. This view ostensibly explains why females do not sing-many of the neural populations and pathways that make up the male song control network appear rudimentary or even missing in females. For example, classic studies of vocal-premotor cortex (HVC, acronym is name) in male zebra finches identified prominent efferent pathways from HVC to vocal-motor cortex (RA, robust nucleus of the arcopallium) and from HVC to the avian basal ganglia (Area X). In females, by comparison, the efferent targets of HVC were thought to be only partially innervated by HVC axons (RA) or absent (Area X). Here, using a novel visually guided surgical approach to target tracer injections with precision, we mapped the extrinsic connectivity of the adult female HVC. We find that female HVC shows a mostly male-typical pattern of afferent and efferent connectivity, including robust HVC innervation of RA and Area X. As noted by earlier investigators, we find large sex differences in the volume of many regions that control male singing (male > female). However, sex differences in volume were diminished in regions that convey ascending afferent input to HVC. Our findings do not support a vestigial interpretation of the song control network in females. Instead, our findings support the emerging view that the song control network may have an altogether different function in nonsinging females.

Using Online Images to Teach Quantitative Skills via Comparative Neuroanatomy: Applying the Directives of Vision and Change

Vision and Change calls for increasing the quantitative skills of biology majors, which includes neuroscience majors. Accordingly, we have devised a module to give students practice at regression analyses, covariance, and ANOVA. This module consists of a quantitative comparative neuroanatomy lab in which students explore the size of the hippocampus relative to the brain in 62 different mammalian species-from an anteater to a zebu. We utilize a digital image library (with appropriate metadata) allowing students to quantify the size of the hippocampus as well as obtain an index of the size of the brain in these various species. Students then answer the following questions: (1) Do brains scale with body size? (2) Does the hippocampus scale with brain size? (3) If we control for body size, does the hippocampus still scale with brain size? (4) How does the hippocampus change as a proportion of brain size? (5) Is the proportional scaling of the hippocampus different among primates, carnivores, and other mammals? (6) Do the data provide evidence for mosaic or concerted evolution? Measures of the pedagogical efficacy showed clear and significant gains on a PreTest vs PostTest assessment of material related to the module. An open ended qualitative measure revealed students' perception of the purposes of the module, which were consistent with the learning goals. This module utilizes open access digital resources and can be performed at any institution. All the materials or links to online resources can be found at https://mdcune.psych.ucla.edu/modules/cna.

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.

Beyond aggression: Androgen-receptor blockade modulates social interaction in wild meerkats

In male vertebrates, androgens are inextricably linked to reproduction, social dominance, and aggression, often at the cost of paternal investment or prosociality. Testosterone is invoked to explain rank-related reproductive differences, but its role within a status class, particularly among subordinates, is underappreciated. Recent evidence, especially for monogamous and cooperatively breeding species, suggests broader androgenic mediation of adult social interaction. We explored the actions of androgens in subordinate, male members of a cooperatively breeding species, the meerkat (Suricata suricatta). Although male meerkats show no rank-related testosterone differences, subordinate helpers rarely reproduce. We blocked androgen receptors, in the field, by treating subordinate males with the antiandrogen, flutamide. We monitored androgen concentrations (via baseline serum and time-sequential fecal sampling) and recorded behavior within their groups (via focal observation). Relative to controls, flutamide-treated animals initiated less and received more high-intensity aggression (biting, threatening, feeding competition), engaged in more prosocial behavior (social sniffing, grooming, huddling), and less frequently initiated play or assumed a 'dominant' role during play, revealing significant androgenic effects across a broad range of social behavior. By contrast, guarding or vigilance and measures of olfactory and vocal communication in subordinate males appeared unaffected by flutamide treatment. Thus, androgens in male meerkat helpers are aligned with the traditional trade-off between promoting reproductive and aggressive behavior at a cost to affiliation. Our findings, based on rare endocrine manipulation in wild mammals, show a more pervasive role for androgens in adult social behavior than is often recognized, with possible relevance for understanding tradeoffs in cooperative systems.

Masculinisation of the zebra finch song system: roles of oestradiol and the Z-chromosome gene tubulin-specific chaperone protein A

Robust sex differences in brain and behaviour exist in zebra finches. Only males sing, and forebrain song control regions are more developed in males. The factors driving these differences are not clear, although numerous experiments have shown that oestradiol (E2 ) administered to female hatchlings partially masculinises brain and behaviour. Recent studies suggest that an increased expression of Z-chromosome genes in males (ZZ; females: ZW) might also play a role. The Z-gene tubulin-specific chaperone A (TBCA) exhibits increased expression in the lateral magnocellular nucleus of the anterior nidopallium (LMAN) of juvenile males compared to females; TBCA+ cells project to the robust nucleus of the arcopallium (RA). In the present study, we investigated the role of TBCA and tested hypotheses with respect to the interactive or additive effects of E2 and TBCA. We first examined whether E2 in hatchling zebra finches modulates TBCA expression in the LMAN. It affected neither the mRNA, nor protein in either sex. We then unilaterally delivered TBCA small interfering (si)RNA to the LMAN of developing females treated with E2 or vehicle and males treated with the aromatase inhibitor, fadrozole, or its control. In both sexes, decreasing TBCA in LMAN reduced RA cell number, cell size and volume. It also decreased LMAN volume in females. Fadrozole in males increased LMAN volume and RA cell size. TBCA siRNA delivered to the LMAN also decreased the projection from this brain region to the RA, as indicated by anterograde tract tracing. The results suggest that TBCA is involved in masculinising the song system. However, because no interactions between the siRNA and hormone manipulations were detected, TBCA does not appear to modulate effects of E2 in the zebra finch song circuit.

17β-estradiol regulates the sexually dimorphic expression of BDNF and TrkB proteins in the song system of juvenile zebra finches

Mature brain derived neurotrophic factor (BDNF) plays critical roles in development of brain structure and function, including neurogenesis, axon growth, cell survival and processes associated with learning. Expression of this peptide is regulated by estradiol (E2). The zebra finch song system is sexually dimorphic - only males sing and the brain regions controlling song are larger and have more cells in males compared to females. Masculinization of this system is partially mediated by E2, and earlier work suggests that BDNF with its high affinity receptor TrkB may also influence this development. The present study evaluated expression of multiple forms of both BDNF and TrkB in the developing song system in juvenile males and females treated with E2 or a vehicle control. Using immunohistochemistry and Western blot analysis, BDNF was detected across the song nuclei of 25-day-old birds. Westerns allowed the pro- and mature forms of BDNF to be individually identified, and proBDNF to be quantified. Several statistically significant effects of sex existed in both the estimated total number of BDNF+ cells and relative concentration of proBDNF, varying across the regions and methodologies. E2 modulated BDNF expression, although the specific nature of the regulation depended on brain region, sex and the technique used. Similarly, TrkB (both truncated and full-length isoforms) was detected by Western blot in the song system of juveniles of both sexes, and expression was regulated by E2. In the context of earlier research on these molecules in the developing song system, this work provides a critical step in describing specific forms of BDNF and TrkB, and how they can be mediated by sex and E2. As individual isoforms of each can have opposing effects on mechanisms, such as cell survival, it will now be important to investigate in depth their specific functions in song system maturation.

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.

Hormonal organization and activation: evolutionary implications and questions

Comparative endocrinology is a fascinating field of science in part because it addresses both ultimate and proximate causation. Research on sexual dimorphism and sexual differentiation has excellent potential for this kind of integration. Vertebrate comparative endocrinologists have made many important discoveries about the role of genes and sex steroid hormones in the organization and activation of sexually differentiated behavior, brain function, anatomy and physiology. In addition to taxonomically general principles and conserved features, there is also striking diversity in sexual differentiation processes. Much of the evolutionary basis of this diversity (its phylogenetic history and adaptive functions) is not well understood. A set of questions is raised to illustrate this point, with an emphasis on mechanisms of sexual dimorphism in body size and ornamentation, sexual differentiation of avian behavior, particularly in Japanese quail and zebra finches, and the puzzle of the phylogenetic distribution of vertebrate sex determining mechanisms. Applying a comparative approach grounded in established phylogenies and concepts from evolutionary developmental biology such as developmental modules holds promise for generating and testing new hypotheses and eventually answering some of these questions.

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

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

Neuroendocrine contributions to sexual partner preference in birds

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

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

Background

Steroids affect many tissues, including the brain. In the zebra finch, the estrogenic steroid estradiol (E₂) is especially effective at promoting growth of the neural circuit specialized for song. In this species, only the males sing and they have a much larger and more interconnected song circuit than females. Thus, it was surprising that the gene for 17β-hydroxysteroid dehydrogenase type 4 (HSD17B4), an enzyme that converts E₂ to a less potent estrogen, had been mapped to the Z sex chromosome. As a consequence, it was likely that HSD17B4 was differentially expressed in males (ZZ) and females (ZW) because dosage compensation of Z chromosome genes is incomplete in birds. If a higher abundance of HSD17B4 mRNA in males than females was translated into functional enzyme in the brain, then contrary to expectation, males could produce less E₂ in their brains than females.

Results

Here, we used molecular and biochemical techniques to confirm the HSD17B4 Z chromosome location in the zebra finch and to determine that HSD17B4 mRNA and activity were detectable in the early developing and adult brain. As expected, HSD17B4 mRNA expression levels were higher in males compared to females. This provides further evidence of the incomplete Z chromosome inactivation mechanisms in birds. We detected HSD17B4 mRNA in regions that suggested a role for this enzyme in the early organization and adult function of song nuclei. We did not, however, detect significant sex differences in HSD17B4 activity levels in the adult brain.

Conclusions

Our results demonstrate that the HSD17B4 gene is expressed and active in the zebra finch brain as an E₂ metabolizing enzyme, but that dosage compensation of this Z-linked gene may occur via post-transcriptional mechanisms.

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.