Distribution and onset of retinaldehyde dehydrogenase (zRalDH) expression in zebra finch brain: Lack of sex difference in HVC and RA at early posthatch ages

Urotensin-related gene transcripts mark developmental emergence of the male forebrain vocal control system in songbirds

Songbirds communicate through learned vocalizations, using a forebrain circuit with convergent similarity to vocal-control circuitry in humans. This circuit is incomplete in female zebra finches, hence only males sing. We show that the UTS2B gene, encoding Urotensin-Related Peptide (URP), is uniquely expressed in a key pre-motor vocal nucleus (HVC), and specifically marks the neurons that form a male-specific projection that encodes timing features of learned song. UTS2B-expressing cells appear early in males, prior to projection formation, but are not observed in the female nucleus. We find no expression evidence for canonical receptors within the vocal circuit, suggesting either signalling to other brain regions via diffusion or transduction through other receptor systems. Urotensins have not previously been implicated in vocal control, but we find an annotation in Allen Human Brain Atlas of increased UTS2B expression within portions of human inferior frontal cortex implicated in human speech and singing. Thus UTS2B (URP) is a novel neural marker that may have conserved functions for vocal communication.

Dynamic gene expression in the song system of zebra finches during the song learning period

The brain circuitry that controls song learning and production undergoes marked changes in morphology and connectivity during the song learning period in juvenile zebra finches, in parallel to the acquisition, practice and refinement of song. Yet, the genetic programs and timing of regulatory change that establish the neuronal connectivity and plasticity during this critical learning period remain largely undetermined. To address this question, we used in situ hybridization to compare the expression patterns of a set of 30 known robust molecular markers of HVC and/or area X, major telencephalic song nuclei, between adult and juvenile male zebra finches at different ages during development (20, 35, 50 days post-hatch, dph). We found that several of the genes examined undergo substantial changes in expression within HVC or its surrounds, and/or in other song nuclei. They fit into broad patterns of regulation, including those whose expression within HVC during this period increases (COL12A1, COL 21A1, MPZL1, PVALB, and CXCR7) or decreases (e.g., KCNT2, SAP30L), as well as some that show decreased expression in the surrounding tissue with little change within song nuclei (e.g. SV2B, TAC1). These results reveal a broad range of molecular changes that occur in the song system in concert with the song learning period. Some of the genes and pathways identified are potential modulators of the developmental changes associated with the emergence of the adult properties of the song control system, and/or the acquisition of learned vocalizations in songbirds.

Long-distance retinoid signaling in the zebra finch brain

All-trans retinoic acid (ATRA), the main active metabolite of vitamin A, is a powerful signaling molecule that regulates large-scale morphogenetic processes during vertebrate embryonic development, but is also involved post-natally in regulating neural plasticity and cognition. In songbirds, it plays an important role in the maturation of learned song. The distribution of the ATRA-synthesizing enzyme, zRalDH, and of ATRA receptors (RARs) have been described, but information on the distribution of other components of the retinoid signaling pathway is still lacking. To address this gap, we have determined the expression patterns of two obligatory RAR co-receptors, the retinoid X receptors (RXR) α and γ, and of the three ATRA-degrading cytochromes CYP26A1, CYP26B1, and CYP26C1. We have also studied the distribution of zRalDH protein using immunohistochemistry, and generated a refined map of ATRA localization, using a modified reporter cell assay to examine entire brain sections. Our results show that (1) ATRA is more broadly distributed in the brain than previously predicted by the spatially restricted distribution of zRalDH transcripts. This could be due to long-range transport of zRalDH enzyme between different nuclei of the song system: Experimental lesions of putative zRalDH peptide source regions diminish ATRA-induced transcription in target regions. (2) Four telencephalic song nuclei express different and specific subsets of retinoid-related receptors and could be targets of retinoid regulation; in the case of the lateral magnocellular nucleus of the anterior nidopallium (lMAN), receptor expression is dynamically regulated in a circadian and age-dependent manner. (3) High-order auditory areas exhibit a complex distribution of transcripts representing ATRA synthesizing and degrading enzymes and could also be a target of retinoid signaling. Together, our survey across multiple connected song nuclei and auditory brain regions underscores the prominent role of retinoid signaling in modulating the circuitry that underlies the acquisition and production of learned vocalizations.

Organization and development of zebra finch HVC and paraHVC based on expression of zRalDH, an enzyme associated with retinoic acid production

The zRalDH gene encodes an aldehyde dehydrogenase associated with the conversion of retinaldehyde (the main vitamin A metabolite) into retinoic acid and its expression is highly enriched in the song control system of adult zebra finches (Taeniopygia guttata). Within song control nucleus HVC, zRalDH is specifically expressed in the neurons that project to area X of the striatum. It is also expressed in paraHVC, commonly considered a medial extension of HVC that is closely associated with auditory areas in the caudomedial telencephalon. Here we used in situ hybridization to generate a detailed analysis of HVC and paraHVC based on expression of zRalDH for adult zebra finches of both sexes and for males during the song-learning period. We demonstrate that the distribution of zRalDH-positive cells can be used for accurate assessments of HVC and paraHVC in adult and juvenile males. We describe marked developmental changes in the numbers of zRalDH-expressing cells in HVC and paraHVC, reaching a peak at day 50 posthatch, an effect potentially due to dynamic changes in the population of X-projecting cells in HVC. We also show that zRalDH-expressing cells in adult females, although much less numerous than in males, have a surprisingly broad distribution along the medial-to-lateral extent of HVC, but are lacking where paraHVC is found in adult males. Our study thus contributes to our understanding of the nuclear organization of the song system and the dynamics of its developmental changes during the song-learning period.

Sex- and age-related differences in ribosomal proteins L17 and L37, as well as androgen receptor protein, in the song control system of zebra finches

The zebra finch song system is sexually dimorphic--only males sing, and the morphology of forebrain regions controlling the learning and production of this song is greatly enhanced in males compared to females. Masculinization appears to involve effects of steroid hormones as well as other factors, perhaps including the expression of sex chromosome genes (males: ZZ, females: ZW). The present study investigated three proteins--two encoded by Z-linked genes, ribosomal proteins L17 and L37 (RPL17 and RPL37), including their co-localization with androgen receptor (AR), from post-hatching day 25 to adulthood. Extensive co-expression of AR with the ribosomal proteins was detected in the three song nuclei investigated (HVC, robust nucleus of the arcopallium (RA), and Area X) across these ages. In general, more cells expressed each of these proteins in males compared to females, and the sex differences increased as animals matured. Specific patterns differed across regions and between RPL17 and RPL37, which suggest potential roles of one or both of these proteins in the incorporation and/or differentiation of song system cells.

Conservation and expression of IQ-domain-containing calpacitin gene products (neuromodulin/GAP-43, neurogranin/RC3) in the adult and developing oscine song control system

Songbirds are appreciated for the insights they provide into regulated neural plasticity. Here, we describe the comparative analysis and brain expression of two gene sequences encoding probable regulators of synaptic plasticity in songbirds: neuromodulin (GAP-43) and neurogranin (RC3). Both are members of the calpacitin family and share a distinctive conserved core domain that mediates interactions between calcium, calmodulin, and protein kinase C signaling pathways. Comparative sequence analysis is consistent with known phylogenetic relationships, with songbirds most closely related to chicken and progressively more distant from mammals and fish. The C-terminus of neurogranin is different in birds and mammals, and antibodies to the protein reveal high expression in adult zebra finches in cerebellar Purkinje cells, which has not been observed in other species. RNAs for both proteins are generally abundant in the telencephalon yet markedly reduced in certain nuclei of the song control system in adult canaries and zebra finches: neuromodulin RNA is very low in RA and HVC (relative to the surrounding pallial areas), whereas neurogranin RNA is conspicuously low in Area X (relative to surrounding striatum). In both cases, this selective downregulation develops in the zebra finch during the juvenile song learning period, 25-45 days after hatching. These results suggest molecular parallels to the robust stability of the adult avian song control circuit.

The neuroendocrine control of sex specific behavior in vertebrates: lessons from mammals and birds

The question of how sex differences in behavior among vertebrates emerge and are expressed has been the topic of intense study for over 50 years. Convergent evidence from birds and mammals, primarily rodents, has provided certain common principles while highlighting other species-specific properties. The importance of early hormonal effects on the developing brain to adult behavioral profile is pervasive throughout the vertebrate phyla and assures that brain sex phenotype will match gonadal phenotype. Variation in the magnitude of differences between males and females in sexual behavior, parenting and aggression are influenced by environmental and physiological parameters. Recent advances in the cellular and molecular mechanisms of steroid hormones in both organizing and activating neural circuits to control behavior reveal a wide variety of effector pathways and emphasize how much we have to learn.

The sexually dimorphic expression of L7/SPA, an estrogen receptor coactivator, in zebra finch telencephalon

Sex differences in the zebra finch (Taeniopygia guttata) brain are robust and include differences in morphology (song control nuclei in males are significantly larger) and behavior (only males sing courtship songs). In zebra finches, hormonal manipulations during development fail to reverse sex differences in song nuclei size and suggest that the classical model of sexual differentiation is incomplete for birds. Coactivators act to initiate transcriptional activity of steroid receptors, and may help explain why hormonal manipulations alone are not sufficient to demasculinize the male zebra finch brain. The present study investigated the expression and localization of L7/SPA (an estrogen receptor coactivator) mRNA and protein expression across the development of zebra finch song nuclei from males and females collected on P1 (song nuclei not yet formed), P10 (posthatch day 10, song nuclei formed), P30 (30 days posthatch, sexually immature but song nuclei formed and birds learning to sing), and adult birds (older than 65 days and sexually mature). Northern blot analysis showed a significant sex difference in P1 and adult L7/SPA mRNA expression while Western blot analysis also showed enhanced expression in the male brain at all age points. Both in situ hybridization and immunohistochemistry demonstrated that L7/SPA mRNA and protein were located in the song nuclei as well as expressed globally. Elevated coactivator expression may be a possible mechanism controlling the development of male song control nuclei, and coactivators such as L7/SPA may be important regulators of the masculinizing effects of estradiol on brain sexual differentiation.

Steroidogenic enzymes along the ventricular proliferative zone in the developing songbird brain

Neural development requires regulation and coordination of the differentiation, migration, and survival of newly divided cells, most of which derive from the region surrounding the lateral ventricles. While many factors are involved in these maturational processes, studies of cell proliferation and neurogenesis in songbirds indicate that sex steroids may provide crucial cues to newly divided cells and may be fundamental to the organization of a specific neural circuit, the song system. In the case of the zebra finch, steroids that impact song system masculinization are most likely not synthesized from the gonads but from the brain, and evidence is mounting that both developing and adult zebra finches have the capacity for neurosteroidogenesis. Therefore, we hypothesized that during early development, all of the genes required for de novo sex steroid synthesis would be expressed in regions that would indicate a role for neurosteroids in neural organization. We found that the genes necessary for de novo neurosteroid synthesis at posthatch day 1 (P1) and P5 show a broad expression distribution. Most strikingly, the spatial distribution of expression for all of the genes necessary for androgen synthesis is similar to the previously described pattern of proliferating neuronal precursors along the lateral border of the lateral ventricle. Due to the increasing evidence for neurosteroid action on multiple cell traits, it may be that locally synthesized neurosteroids impact cells along the proliferative zone to influence early events in neural development generally and song system masculinization specifically.

Expression of NGF and trkA mRNA in song control and other regions of the zebra finch brain

We used in situ hybridization to measure the expression of NGF and trkA mRNA in the zebra finch brain at posthatch day 11 (P11), P25, and in adulthood. Expression of NGF and trkA was restricted to specific areas of the telencephalon in the adult zebra finch brain. Interestingly the expression of NGF and trkA overlapped in most brain regions, suggesting that NGF acts at sites close to cells that synthesize it. In song regions of adults, both NGF and trkA were clearly expressed in lMAN, HVC, and RA in males and in lMAN and RA in females. At P11, NGF and trkA mRNA were detected only in RA in both sexes. At P25, when sex differences in lMAN and RA begin to emerge, NGF mRNA was expressed in lMAN and RA in both sexes and trkA was detected at low levels in lMAN in both sexes. Whereas the level of trkA expression in RA of males at P25 was consistently low but detectable, expression in females was not detected. The volume of RA defined by NGF was significantly larger in males than females at P25. We also found a tendency for the intensity of NGF in RA to be higher in males than in females at P25, although the difference was not statistically significant. The presence of NGF and trkA mRNA in RA and lMAN at P25 suggests that they may participate in sexually dimorphic neural development of RA and lMAN, possibly by participating in sex-specific cell survival.

The distribution of expression of doublecortin (DCX) mRNA and protein in the zebra finch brain

Using in situ hybridization, we measured the distribution of expression of doublecortin (DCX), a microtubule-associated protein, in zebra finch adult and nestling (P9-11) brains. In adult brain, DCX mRNA was detected mainly in the mesopallium (M), medial striatum (MSt), septum, Area X, diencephalon, telencephalic subventricular zone (SVZ), and Purkinje cells in the cerebellum. The expression at posthatch day 9 (P9) was heavy in almost the entire telencephalon and showed heavier expression in SVZ and song regions such as the high vocal center (HVC) and the robust nucleus of arcopallium (RA). Outside of the telencephalon at P9, we found distinct label in nucleus ovoidalis (OV), nucleus spiriformis lateralis (SpL), and nucleus subpretectalis (SP) in the midbrain, almost the entire diencephalon including nucleus dorsomedialis posterior thalami (DMP), stratum griseum et fibrosum superficiale (SGF) in optic tectum, and Purkinje cells in cerebellum. Most of the heavily labeled areas by in situ hybridization overlapped with immunohistochemical staining for DCX, indicating that DCX mRNA is probably translated into protein in those regions. No sex difference was found in DCX expression at P9 or in the adult except that Area X was labeled only in the adult male. The intensity of expression in the adult was significantly lower than that at P9, which suggests a particular role for DCX in early song bird brain development. If DCX is predominantly expressed in migrating neurons, as suggested from studies in mammals, the present results offer no evidence for a sex difference in neuronal migration.