Increased expression of endogenous biotin, but not BDNF, in telencephalic song regions during zebra finch vocal learning

Developmental changes in BDNF protein in the song control nuclei of zebra finches

The zebra finch song system provides an excellent model to study the mechanisms underlying the development of sex difference in brain structure and function. Only male zebra finches sing and the brain nuclei controlling song learning and production are considerably larger than in females. Sexual differentiation may in part be regulated by estrogen, but other molecules including neurotrophic factors likely also affect masculinization. Brain derived neurotrophic factor (BDNF) plays a crucial role in numerous aspects of vertebrate brain development and function, including neurogenesis, cell survival, growth of axonal projections, synaptogenesis and processes linked to learning and memory. The current study investigated the expression of BDNF protein in juvenile males and females at four ages, as well as in adults, to begin to evaluate the potential roles of endogenous BDNF in particular stages of structural and functional development of the song system. In both HVC and the robust nucleus of the arcopallium (RA), males had more BDNF+ cells than females. The number of immunopositive cells increased in males and decreased in females as they matured, in a pattern generally consistent with a role for BDNF in sensorimotor integration of song learning. In addition, in HVC (but not RA) the ratio of mature BDNF compared to its precursor proBDNF was greater in adult males than those at post-hatching day 25, indicating a region-specific shift in the relative availability of the two forms. Collectively, the data suggest that changes in BDNF protein expression across development may be associated with song system maturation, particularly during the sensorimotor integration of masculine vocalizations.

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.

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.

Biotin--a regulator of gene expression

The role of biotin as the prosthetic group of the four biotin-dependent carboxylases in higher organisms is well recognized. Based on the roles of these carboxylases in metabolism, the requirement of biotin for cell viability, growth and differentiation was established. Biotin seems to have a role in cell functions other than as the prosthetic group of biotin enzymes. Biotin seems to influence processes such as the proliferation of the mesenchyme, spermatogenesis and song-bird vocalization. A direct effect of biotin, at the transcriptional level, has been shown for the key enzymes of glucose metabolism. Glucokinase, a key glycolytic enzyme, and phosphoenolpyruvate carboxykinase (PEPCK), a key gluconeogenic enzyme, are regulated in opposite directions by biotin in a manner similar to the action of insulin.

Acute injections of brain-derived neurotrophic factor in a vocal premotor nucleus reversibly disrupt adult birdsong stability and trigger syllable deletion

Behavioral variability serves an essential role in motor learning by enabling sensory feedback to select those motor patterns that minimize error. Birds use auditory feedback to learn how to sing, and their songs lose variability and become highly stereotyped, or crystallized, at the end of a sensitive period for sensorimotor learning. The molecular cues that regulate song variability are not well understood. In other systems, neurotrophins, and brain-derived neurotrophic factor (BDNF) in particular, can mediate various forms of neural plasticity, including sensitive period neural circuit plasticity and activity-dependent synapse formation, and may also influence learning and memory. Here, we have tested the hypothesis that neurotrophin expression in the robust nucleus of the arcopallium (RA), the telencephalic output controlling song, regulates song variability. BDNF and its receptor trkB are expressed in RA, and BDNF expression in RA appears to be highest in juveniles, when song is most variable and plastic, and synapse density highest. Thus, song variability and synaptic connectivity could be enhanced by augmented expression of BDNF in RA. In support of this idea, we found that BDNF injections into the adult RA induced the re-expression of juvenile-like phenotypes, including song variability and an increased synaptic density in RA. Furthermore, BDNF treatment also induced vocal plasticity, characterized by syllable deletions and persistent changes to the song patterns. These results suggest that endogenous BDNF could be a molecular regulator of the song variability essential to vocal plasticity and, ultimately, to song learning.

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.

Biotin is endogenously expressed in select regions of the rat central nervous system

The vitamin biotin is an endogenous molecule that acts as an important cofactor for several carboxylases in the citric acid cycle. Disorders of biotin metabolism produce neurological symptoms that range from ataxia to sensory loss, suggesting the presence of biotin in specific functional systems of the CNS. Although biotin has been described in some cells of nonmammalian nervous systems, the distribution of biotin in mammalian CNS is virtually unknown. We report the presence of biotin in select regions of rat CNS, as revealed with a monoclonal antibody directed against biotin and with avidin- and streptavidin-conjugated labels. Detectable levels of biotin were primarily found caudal to the diencephalon, with greatest expression in the cerebellar motor system and several brainstem auditory nuclei. Biotin was found as a somatic label in cerebellar Purkinje cells, in cell bodies and proximal dendrites of cerebellar deep nuclear neurons, and in red nuclear neurons. Biotin was detected in cells of the spiral ganglion, somata and proximal dendrites of cells in the cochlear nuclei, superior olivary nuclei, medial nucleus of the trapezoid body, and nucleus of the lateral lemniscus. Biotin was further found in pontine nuclei and fiber tracts, the substantia nigra pars reticulata, lateral mammillary nucleus, and a small number of hippocampal interneurons. Biotin was detected in glial cells of major tract systems throughout the brain but was most prominent in tracts of the hindbrain. Biotin is thus expressed in select regions of rat CNS with a distribution that correlates to the known clinical sequelae associated with biotin deficiencies.

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.

Endogenous biotin staining in a subset of spinal neuronal cell bodies: a potential confounding factor for neuroanatomical studies

Biotinylated compounds are commonly used to label neuronal cell bodies via intracellular filling or retrograde tracing. Endogenous concentrations of biotin within a subset of neuronal cell bodies would pose a problem for interpreting such experiments. Here I report that a subset of turtle spinal cord neuronal cell bodies strongly stains for biotin, using the avidin-biotin-horseradish peroxidase (ABC) reaction, in the absence of any exogenous biotinylated compound.

Age and sex differences in mitotic activity within the zebra finch telencephalon

Brain regions associated with song learning in zebra finches are larger and contain more neurons in males than females. Differences in cell proliferation, migration, survival, and specification may all contribute to the divergent development of the song-control system in developing birds. This study quantified levels of cell proliferation within the telencephalic ventricular zone (VZ) of juvenile and adult birds to look for both age and sex differences in mitotic activity that might contribute to the construction of song-control circuits. A single pulse of [(3)H]thymidine was administered to juveniles and adults of both sexes, and animals were killed 2 hr later. Analysis of thymidine labeling within the telencephalic VZ at the levels of area X, the anterior commissure, and high vocal center (HVC) revealed two major findings: (1) levels of mitotic activity decreased as a function of age in both males and females because of a reduction in the number of dividing cells within the VZ, and (2) sex differences in thymidine labeling occurred in restricted, localized segments of the VZ at the levels of area X and the anterior commissure in juveniles but not adults. Thus, overall proliferative activity decreases as birds mature, and the incidence of cell division in all regions of the VZ becomes equivalent in both sexes, such that no regions of sexually dimorphic proliferation are evident by adulthood. These data suggest that regions of sexually dimorphic proliferation within the VZ may contain precursor cells that give rise to song-control neurons, such that higher rates of mitotic activity in juvenile males could contribute to the growth of song-control nuclei such as HVC and area X.

Zebra finch sexual differentiation: the aromatization hypothesis revisited

Zebra finches have emerged as an outstanding model system for the investigation of the mechanisms regulating brain and behavior. Their song system has proven especially useful, as the function of discrete anatomical regions have been identified, and striking parallels exist between the morphology of these regions and the level of their function in males and females. That is, the structures are substantially more developed in males, who sing, compared to females, who do not. These parallels extend from higher (telencephalic) centers to the brainstem motor nucleus that innervates the muscles of the vocal organ. Other dimorphic aspects of reproduction in the zebra finch, such as copulatory behaviors and sexual partner preference, however, are not associated with known sex differences in anatomy. In many species, sex differences in neural and peripheral structures and behavior are regulated by secretions from the gonads, which of course are sexually dimorphic themselves. In birds, sex differences at all of these levels (gonad, brain, and behavior) can be mediated by steroid hormones. However, it is not entirely clear that gonadal secretions normally participate at all of the levels. This paper reviews the evidence relating to the role of gonadal steroids in the sexual differentiation of reproductive behaviors and the central and peripheral structures known to regulate them in zebra finches, with a focus on estradiol, which has been most extensively studied in the masculinization of song system morphology and function.

Functional changes in field L complex during song development of juvenile male zebra finches

The field L complex is the highest station of the ascending auditory pathway and is thought to be the input stage of auditory information into the song system in birds. Multi-unit recordings were performed in awake, socially reared zebra finches, 30 and 60 days of age. The responses of the field L complex to synthetic and natural stimuli during important periods of song learning were investigated. According to neural responses in field L, three different functional areas could be distinguished, NA-L(30), NA2b(30) and NA2c(30,) in 30 days old birds. In 60 days old birds five different functional areas, NA-L(60), NA2a(60), NA2b(60), NA2c(60) and NA3(60), were recognised. Especially, NA-L increases its functional volume between the developmental stages. The different areas showed already mature neuronal response behaviours. No preference for a certain song type could be found at all ages. The incomplete functional organisation of the field L complex in young birds (30 days) is a possible reason for the nonselectivity in the song system at this age.

Post-transcriptional regulation of zenk expression associated with zebra finch vocal development

In the male zebra finch, highly variable juvenile song and stereotyped adult song induce mRNA expression of the immediate early gene zenk in telencephalon. However, the functional consequences of this behavior-driven gene expression remain unknown. Here we characterize the developmental expression of zenk mRNA and protein in two forebrain song regions (HVC, the higher vocal center, and RA, the robust nucleus of the archistriatum). In HVC, singing results in similar percentages of cells producing zenk mRNA and zenk protein at different stages of vocal development. Similarly, song behavior at all stages of vocal development induces a comparable percentage of RA cells expressing zenk mRNA. However, the percentage of RA zenk immunoreactive cells is low during early vocal learning, increasing only as the vocal pattern matures. Early induction of a stereotyped vocal pattern in juvenile birds is associated with increased zenk immunoreactivity in RA, indicating that it is the form of the behavior (and not the age of the bird) that correlates with changes in zenk immunoreactivity. Together, our findings reveal a previously unrecognized relationship between behavioral development and post-transcriptional gene regulation.