A relationship between behavior, neurotrophin expression, and new neuron survival

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.

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

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

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.

Adult neurogenesis is associated with the maintenance of a stereotyped, learned motor behavior

Adult neurogenesis is thought to provide neural plasticity used in forming and storing new memories. Here we show a novel relationship between numbers of new neurons and the stability of a previously learned motor pattern. In the adult zebra finch, new projection neurons are added to the nucleus HVC and become part of the motor pathway for producing learned song. However, new song learning occurs only in juveniles and the behavioral impact of adding new neurons to HVC throughout life is unclear. We report that song changes after deafening are inversely correlated with the number of new neurons added to HVC, suggesting that adult neurogenesis in this context may contribute to behavioral stability. More broadly, we propose that new neuron function may depend on the site of integration and can vary as widely as promoting, or restricting, behavioral plasticity.

Sculpting reproductive circuits: relationships among hormones, morphology and behavior in anole lizards

Morphology parallels function on a variety of levels in reproductive circuits in anole lizards, as in many vertebrate groups. For example, across species within the anole genus the muscle fibers regulating extension of a throat fan used in courtship are larger in males than females. Endocrine factors controlling behavior and morphology have been studied in detail in one species, the green anole (Anolis carolinensis). This review briefly describes the results that have been obtained and highlights key areas for future investigation that will provide insights on mechanisms from a comparative perspective.

Seasonal changes in patterns of gene expression in avian song control brain regions

Photoperiod and hormonal cues drive dramatic seasonal changes in structure and function of the avian song control system. Little is known, however, about the patterns of gene expression associated with seasonal changes. Here we address this issue by altering the hormonal and photoperiodic conditions in seasonally-breeding Gambel's white-crowned sparrows and extracting RNA from the telencephalic song control nuclei HVC and RA across multiple time points that capture different stages of growth and regression. We chose HVC and RA because while both nuclei change in volume across seasons, the cellular mechanisms underlying these changes differ. We thus hypothesized that different genes would be expressed between HVC and RA. We tested this by using the extracted RNA to perform a cDNA microarray hybridization developed by the SoNG initiative. We then validated these results using qRT-PCR. We found that 363 genes varied by more than 1.5 fold (>log₂ 0.585) in expression in HVC and/or RA. Supporting our hypothesis, only 59 of these 363 genes were found to vary in both nuclei, while 132 gene expression changes were HVC specific and 172 were RA specific. We then assigned many of these genes to functional categories relevant to the different mechanisms underlying seasonal change in HVC and RA, including neurogenesis, apoptosis, cell growth, dendrite arborization and axonal growth, angiogenesis, endocrinology, growth factors, and electrophysiology. This revealed categorical differences in the kinds of genes regulated in HVC and RA. These results show that different molecular programs underlie seasonal changes in HVC and RA, and that gene expression is time specific across different reproductive conditions. Our results provide insights into the complex molecular pathways that underlie adult neural plasticity.

The zebra finch paradox: song is little changed, but number of neurons doubles

New neurons are added to the high vocal center (HVC) of adult males in seasonally breeding songbirds such as the canary (Serinus canaria) that learns new songs in adulthood, and the song sparrow (Melospiza melodia) that does not. In both cases, the new neurons numerically replace others that have died, resulting in a seasonal fluctuation in HVC volume and neuron number. Peaks in neuronal replacement in both species occur in the fall when breeding is over and song is variable. New neurons are added, too, to the HVC of zebra finches (Taeniopygia guttata) that do not learn new songs in adulthood and whose song remains stereotyped throughout the year. Here, we show that, in contrast to the observations in seasonal songbirds, neurons added to the zebra finch HVC are not part of a replacement process. Rather, they lead to a doubling in the number of neurons that project from HVC to the robust nucleus of the arcopallium (RA). As this happens, HVC volume remains constant and the packing density of its neurons increases. The HVC-RA neurons are part of a descending pathway that carries the pattern of learned song; some HVC-RA neurons are also responsive to song playback. The addition of HVC-RA neurons happens in zebra finches housed singly, but becomes more acute if the birds are housed communally. We speculate that new neurons added to the adult HVC may help with the production or perception of learned song, or both.

Socially modulated cell proliferation is independent of gonadal steroid hormones in the brain of the adult green treefrog (Hyla cinerea)

Gonadal steroid hormones have been shown to influence adult neurogenesis in addition to their well-defined role in regulating social behavior. Adult neurogenesis consists of several processes including cell proliferation, which can be studied via 5-bromo-2'-deoxyuridine (BrdU) labeling. In a previous study we found that social stimulation altered both cell proliferation and levels of circulating gonadal steroids, leaving the issue of cause/effect unclear. In this study, we sought to determine whether socially modulated BrdU-labeling depends on gonadal hormone changes. We investigated this using a gonadectomy-implant paradigm and by exposing male and female green treefrogs (Hyla cinerea) to their conspecific chorus or control stimuli (i.e. random tones). Our results indicate that socially modulated cell proliferation occurred independently of gonadal hormone levels; furthermore, neither androgens in males nor estrogen in females increased cell proliferation in the preoptic area (POA) and infundibular hypothalamus, brain regions involved in endocrine regulation and acoustic communication. In fact, elevated estrogen levels decreased cell proliferation in those brain regions in the implanted female. In male frogs, evoked calling behavior was positively correlated with BrdU-labeling in the POA; however, statistical analysis showed that this behavior did not mediate socially induced cell proliferation. These results show that the social modulation of cell proliferation can occur without gonadal hormone involvement in either male or female adult anuran amphibians, and confirms that it is independent of a behavioral response in males.

Adult neuron addition to the zebra finch song motor pathway correlates with the rate and extent of recovery from botox-induced paralysis of the vocal muscles

In adult songbirds, neurons are continually incorporated into the telencephalic nucleus HVC (used as a proper name), a premotor region necessary for the production of learned vocalizations. Previous studies have demonstrated that neuron addition to HVC is highest when song is most variable: in juveniles during song learning, in seasonally singing adults during peaks in plasticity that precede the production of new song components, or during seasonal reestablishment of a previously learned song. These findings suggest that neuron addition provides motor flexibility for the transition from a variable song to a target song. Here we test the association between the quality of song structure and HVC neuron addition by experimentally manipulating syringeal muscle control with Botox, which produces a transient partial paralysis. We show that the quality of song structure covaries with new neuron addition to HVC. Both the magnitude of song distortion and the rate of song recovery after syringeal Botox injections were correlated with the number of new neurons incorporated into HVC. We suggest that the quality of song structure is either a cause or consequence of the number of new neurons added to HVC. Birds with naturally high rates of neuron addition may have had the greatest success in recovering song. Alternatively, or in addition, new neuron survival in the song motor pathway may be regulated by the quality of song-generated feedback as song regains its original stereotyped structure. Present results are the first to show a relationship between peripheral muscle control and adult neuron addition to cortical premotor circuits.

From pattern to purpose: how comparative studies contribute to understanding the function of adult neurogenesis

The study of adult neurogenesis has had an explosion of fruitful growth. Yet numerous uncertainties and challenges persist. Our review begins with a survey of species that show evidence of adult neurogenesis. We then discuss how neurogenesis varies across brain regions and point out that regional specializations can indicate functional adaptations. Lifespan and aging are key life-history traits. Whereas 'adult neurogenesis' is the common term in the literature, it does not reflect the reality of neurogenesis being primarily a 'juvenile' phenomenon. We discuss the sharp decline with age as a universal trait of neurogenesis with inevitable functional consequences. Finally, the main body of the review focuses on the function of neurogenesis in birds and mammals. Selected examples illustrate how our understanding of avian and mammalian neurogenesis can complement each other. It is clear that although the two phyla have some common features, the function of adult neurogenesis may not be similar between them and filling the gaps will help us understand neurogenesis as an evolutionarily conserved trait to meet particular ecological pressures.

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

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

Glucocorticoid receptor blockade inhibits brain cell addition and aggressive signaling in electric fish, Apteronotus leptorhynchus

When animals are under stress, glucocorticoids commonly inhibit adult neurogenesis by acting through glucocorticoid receptors (GRs). However, in some cases, conditions that elevate glucocorticoids promote adult neurogenesis, and the role of glucocorticoid receptors in these circumstances is not well understood. We examined the involvement of GRs in social enhancement of brain cell addition and aggressive signaling in electric fish, Apteronotus leptorhynchus. In this species, long-term social interaction simultaneously elevates plasma cortisol, enhances brain cell addition and increases production of aggressive electrocommunication signals ("chirps"). We implanted isolated and paired fish with capsules containing nothing (controls) or the GR antagonist, RU486, recorded chirp production and locomotion for 7d, and measured the density of newborn cells in the periventricular zone. Compared to isolated controls, paired controls showed elevated chirping in two phases: much higher chirp rates in the first 5h and moderately higher nocturnal rates thereafter. Treating paired fish with RU486 reduced chirp rates in both phases to those of isolated fish, demonstrating that GR activation is crucial for socially induced chirping. Neither RU486 nor social interaction affected locomotion. RU486 treatment to paired fish had a partial effect on cell addition: paired RU486 fish had less cell addition than paired control fish but more than isolated fish. This suggests that cortisol activation of GRs contributes to social enhancement of cell addition but works in parallel with another GR-independent mechanism. RU486 also reduced cell addition in isolated fish, indicating that GRs participate in the regulation of cell addition even when cortisol levels are low.

Developmental changes in the sexually dimorphic expression of secretory carrier membrane protein 1 and its co-localisation with androgen receptor protein in the zebra finch song system

The song system of zebra finches differs dramatically between the sexes in terms of both structure and function. Only males sing and the brain regions regulating the learning and production of this behaviour are far more developed in males than females. Mechanisms regulating sexual differentiation likely include both direct genetic and hormonal processes. Expression of both mRNA and the protein product for secretory carrier membrane protein 1 (SCAMP1), a sex chromosome gene, are increased in the brains of juvenile males compared to females. Here we investigated developmental changes in SCAMP1 containing cells in song nuclei and co-localisation with androgen receptor (AR) protein from post-hatching day 25 through adulthood. Almost all SCAMP1 cells co-expressed AR and approximately half of the AR cells expressed SCAMP1 in the HVC and robust nucleus in the arcopallium (RA) of both sexes and in the Area X of males (which could not be clearly defined in females). In HVC and RA, more single and double-labelled cells were detected in males than females overall, and the sex differences increased as animals matured. The results suggest the potential for interaction of these two proteins in regulating development of brain and/or behaviour.

Newborn GnRH neurons in the adult forebrain of the ring dove

The preoptic area of the hypothalamus is a key area that produces gonadotrophin-releasing hormone (GnRH). In birds, the chicken GnRH-I-form neurons are responsible for the hypothalamus-pituitary-gonadal system, which controls reproduction. In the ring dove, electrolytic lesion in the adult hypothalamus induces neurogenesis. In this study, we determined whether adult neurogenesis is involved in repairing GnRH neurons, specifically by generating newborn cells exhibiting GnRH-I immunoreactive properties. We selectively applied electrolytic lesions to three different regions of the diencephalon, including the preoptic area, which contains GnRH-I neurons, and identified new cells (BrdU-positive cells) that co-labeled with GnRH-I-immunoreactive cells. The BrdU(+)/GnRH(+) double labeled cells were then confirmed with confocal laser analysis. In brains of both male and female ring doves we found new neurons at the lesion site of the preoptic region that were GnRH-I immunoreactive. However, the total number of GnRH neurons in the lesioned brains was less than that of sham-lesioned brains. When two other regions of the diencephalon that contain GnRH-I neurons were damaged, no recruitment of new GnRH-I neurons was detected. The rate of neurogenesis depends on the bird's reproductive phase when the lesion was applied. We found BrdU(+)/GnRH(+) double-labeled cells almost exclusively during the pre-laying phase when birds are engaged in active courtship that leads to egg laying. Our observations suggest that recruitment of GnRH immunoreactive new neurons is restricted to the hypothalamic region and is sensitive to the reproductive stage of the birds.

Sex steroid-induced neuroplasticity and behavioral activation in birds

The brain of adult homeothermic vertebrates exhibits a higher degree of morphological neuroplasticity than previously thought, and this plasticity is especially prominent in birds. In particular, incorporation of new neurons is widespread throughout the adult avian forebrain, and the volumes of specific nuclei vary seasonally in a prominent manner. We review here work on steroid-dependent plasticity in birds, based on two cases: the medial preoptic nucleus (POM) of Japanese quail in relation to male sexual behavior, and nucleus HVC in canaries, which regulates song behavior. In male quail, POM volume changes seasonally, and in castrated subjects testosterone almost doubles POM volume within 2 weeks. Significant volume increases are, however, already observable after 1 day. Steroid receptor coactivator-1 is part of the mechanism mediating these effects. Increases in POM volume reflect changes in cell size or spacing and dendritic branching, but are not associated with an increase in neuron number. In contrast, seasonal changes in HVC volume reflect incorporation of newborn neurons in addition to changes in cell size and spacing. These are induced by treatments with exogenous testosterone or its metabolites. Expression of doublecortin, a microtubule-associated protein, is increased by testosterone in the HVC but not in the adjacent nidopallium, suggesting that neuron production in the subventricular zone, the birthplace of newborn neurons, is not affected. Together, these data illustrate the high degree of plasticity that extends into adulthood and is characteristic of avian brain structures. Many questions still remain concerning the regulation and specific function of this plasticity.

Male bias in the song control system despite female bias in song rate in streak-backed orioles (Icterus pustulatus)

The song control system is a group of discrete interconnected nuclei found in the brains of all songbirds (suborder Passeri). Previous studies have reported a positive relationship between sex differences in song nucleus volumes and sex differences in song behavior across numerous songbird species, with species exhibiting greater sex differences in behavior also exhibiting greater sex differences in the brain. This body of comparative research, however, has failed to incorporate data from a bird species in which females sing more than males. In this study, we examine song nucleus volumes in both sexes of the streak-backed oriole (Icterus pustulatus), a New World blackbird with a female bias in song rate and similar song complexity between the sexes. Results from this neuroanatomical analysis are contrary to what was to be expected from previous research: despite the female bias in song rate, males have a significantly larger HVC and area X song nucleus volumes. Specifically, male HVC was 75% larger than that of females, and male area X was 64% larger than that of females. There was no significant sex difference in the size of the nucleus robustus arcopallialis. The lack of a positive relationship between song nuclei and singing behavior in these orioles demonstrates that our current understanding of song modulation via the song control system may be overly reliant on basic measures such as total volumes.

The relationship of neurogenesis and growth of brain regions to song learning

Song learning, maintenance and production require coordinated activity across multiple auditory, sensory-motor, and neuromuscular structures. Telencephalic components of the sensory-motor circuitry are unique to avian species that engage in song learning. The song system shows protracted development that begins prior to hatching but continues well into adulthood. The staggered developmental timetable for construction of the song system provides clues of subsystems involved in specific stages of song learning and maintenance. Progressive events, including neurogenesis and song system growth, as well as regressive events such as apoptosis and synapse elimination, occur during periods of song learning and the transitions between variable and stereotyped song during both development and adulthood. There is clear evidence that gonadal steroids influence the development of song attributes and shape the underlying neural circuitry. Some aspects of song system development are influenced by sensory, motor and social experience, while other aspects of neural development appear to be experience-independent. Although there are species differences in the extent to which song learning continues into adulthood, growing evidence suggests that despite differences in learning trajectories, adult refinement of song motor control and song maintenance can require remarkable behavioral and neural flexibility reminiscent of sensory-motor learning.

Stress coping stimulates hippocampal neurogenesis in adult monkeys

Coping with intermittent social stress is an essential aspect of living in complex social environments. Coping tends to counteract the deleterious effects of stress and is thought to induce neuroadaptations in corticolimbic brain systems. Here we test this hypothesis in adult squirrel monkey males exposed to intermittent social separations and new pair formations. These manipulations simulate conditions that typically occur in male social associations because of competition for limited access to residency in mixed-sex groups. As evidence of coping, we previously confirmed that cortisol levels initially increase and then are restored to prestress levels within several days of each separation and new pair formation. Follow-up studies with exogenous cortisol further established that feedback regulation of the hypothalamic-pituitary-adrenal axis is not impaired. Now we report that exposure to intermittent social separations and new pair formations increased hippocampal neurogenesis in squirrel monkey males. Hippocampal neurogenesis in rodents contributes to spatial learning performance, and in monkeys we found that spatial learning was enhanced in conditions that increased hippocampal neurogenesis. Corresponding changes were discerned in the expression of genes involved in survival and integration of adult-born granule cells into hippocampal neural circuits. These findings support recent indications that stress coping stimulates hippocampal neurogenesis in adult rodents. Psychotherapies designed to promote stress coping potentially have similar effects in humans with major depression.

Steroidal and gonadal effects on neural cell proliferation in vitro in an adult songbird

Neurogenesis in the adult songbird brain occurs along the ventricular zone (VZ), a specialized cell layer surrounding the lateral ventricles. To examine the acute effects of sex steroids on VZ cell proliferation, male and female adult zebra finch brain slices containing the VZ were exposed to 5-bromo-2'-deoxyuridine-5'-monophosphate (BrdU) in vitro. Slices from one hemisphere served as the control, while contralateral slices were treated with steroids, steroidogenic enzyme inhibitors or gonadal tissue itself. There were no significant effects on VZ cell proliferation in either sexes by acute exposure to 17beta-estradiol (E2), dihydrotestosterone (DHT), a cocktail of four sex steroids, and inhibitors of sex steroid synthesis (aminoglutethimide, ketoconazole, and fadrozole), or by activation of a mitochondrial cholesterol transporter. By contrast, dehydroepiandrosterone (DHEA) suppressed VZ cell proliferation in males, but not females, replicating previous observations involving treatments with corticosterone and RU-486. This suggests that DHEA suppresses proliferation in males via a glucocorticoid receptor-related mechanism. These results suggest that neurosteroidogenesis per se has little effect on acute VZ cell proliferation. Co-incubation with an ovary of female, but not male, slices significantly increased VZ cell proliferation; testicular tissue had no impact on proliferation in males or females. This suggests a role for a non-steroidal ovarian factor on adult female VZ cell proliferation. We also have evidence that previously reported sex-differences in BrdU-labeling along the adult VZ (males>females) result from a more rapid loss of cells in females. Sex differences in steroid action and cell death along the VZ may contribute to the maintenance of the sexually dimorphic song system.

Co-localization of sorting nexin 2 and androgen receptor in the song system of juvenile zebra finches

Mechanisms regulating sexual differentiation of the zebra finch song system appear to include both genetic and hormonal factors. Sorting Nexin 2 (SNX2), which is involved in trafficking proteins between cellular membranes, and androgen receptor (AR) mRNA are both increased in song control nuclei of juvenile males compared to females. Here, in situ hybridization for SNX2 and immunohistochemistry for AR were used to evaluate these sexual dimorphisms in more detail. Estimates of the total number of HVC cells expressing SNX2 and AR, individually as well as together, were greater in 25-day-old males compared to females. The densities of these types of cells were generally also increased in males compared to females in HVC and Area X (or the equivalent portion of the medial striatum in females). On average, more than half of the AR+ cells co-expressed SNX2 in both brain regions. The potential, therefore, exists for both AR and SNX2 to be involved in masculinization of these two brain regions. One possibility is that they, either separately or in conjunction, enhance the action of trophic factors within the brain.

In vivo nicotine exposure in the zebra finch: a promising innovative animal model to use in neurodegenerative disorders related research

Nicotine improves cognitive enhancement and there are indications that neurodegenerative (age-related) cognitive disorders could be treated with nicotine-based drugs. The zebra finch is a well-recognized model to study cognitive functioning; hence this model could be used to study the effects of nicotine in neurodegenerative cognitive disorders. However, nicotine's in vivo physiological and behavioral effects have never been studied in the zebra finch. Here we present the first in vivo nicotine study in zebra finches. We evaluated the dose-response effects of nicotine on locomotor activity, song production, food intake and body weight. A liquid chromatography tandem mass spectrometry method was developed and validated for quantification of nicotine and cotinine in feces. The subcutaneous nicotine drug regiment (0.054-0.54mg/kg) induced physiologically significant values of nicotine and cotinine. The mid (0.18mg/kg) and high (0.54mg/kg) dose of nicotine promoted the development and expression of a sensitized response of song production and locomotor activity. Food intake and body weight were not affected following nicotine exposure. In conclusion, the zebra finch can be used as an innovative animal model not only in nicotine-related research studying cognitive functioning, but also in studies examining nicotine dependence and addictive mechanisms.

Deafening decreases neuronal incorporation in the zebra finch caudomedial nidopallium (NCM)

New neurons formed in the adult brain are incorporated into existing circuits. However, the number of new neurons recruited into a given brain region varies widely depending on the experience of the animal. An emerging general principle is that recruitment and early neuronal survival may be correlated with activity or use of the brain region. Here we show that use-dependent neuronal survival also occurs in the higher order auditory processing region of the songbird caudomedial nidopallium (NCM). We suggest that retention of young neurons may in part be influenced by use of the system without an increased demand for learning or behavioral plasticity.

Brain-derived neurotrophic factor signaling in the HVC is required for testosterone-induced song of female canaries

Testosterone-induced singing in songbirds is thought to involve testosterone-dependent morphological changes that include angiogenesis and neuronal recruitment into the HVC, a central part of the song control circuit. Previous work showed that testosterone induces the production of vascular endothelial growth factor (VEGF) and its receptor (VEGFR2 tyrosine kinase), which in turn leads to an upregulation of brain-derived neurotrophic factor (BDNF) production in HVC endothelial cells. Here we report for the first time that systemic inhibition of the VEGFR2 tyrosine kinase is sufficient to block testosterone-induced song in adult female canaries, despite sustained androgen exposure and the persistence of the effects of testosterone on HVC morphology. Expression of exogenous BDNF in HVC, induced locally by in situ transfection, reversed the VEGFR2 inhibition-mediated blockade of song development, thereby restoring the behavioral phenotype associated with androgen-induced song. The VEGFR2-inhibited, BDNF-treated females developed elaborate male-like song that included large syllable repertoires and high syllable repetition rates, features known to attract females. Importantly, although functionally competent new neurons were recruited to HVC after testosterone treatment, the time course of neuronal addition appeared to follow BDNF-induced song development. These findings indicate that testosterone-associated VEGFR2 activity is required for androgen-induced song in adult songbirds and that the behavioral effects of VEGFR2 inhibition can be rescued by BDNF within the adult HVC.

Neural mechanisms for learned birdsong

Learning by imitation is essential for transmitting many aspects of human culture, including speech, language, art, and music. How the human brain enables imitation remains a mystery, but the underlying neural mechanisms must harness sensory feedback to adaptively modify performance in reference to the object of imitation. Although examples of imitative learning in nonhuman animals are relatively rare, juvenile songbirds learn to sing by copying the song of an adult tutor. The delineation of neural circuits for birdsong raises the promise that this complex form of vocal learning, which bears strong parallels to human speech learning, can be understood in terms of underlying neural mechanisms. This promise is now being more fully realized, with recent experimental advances leading to better understanding of the central motor codes for song and the central mechanisms by which auditory experience modifies song motor commands to enable vocal learning.

Sex-specific modulation of cell proliferation by socially relevant stimuli in the adult green treefrog brain (Hyla cinerea)

Social experience plays an important role in regulating the neural, physiological and hormonal changes that accompany the expression of reproductive behavior in vertebrates. This suite of functions is sexually dimorphic, with different neural control areas preeminent in males and females. In anuran amphibians, social experience comes in the form of acoustic communication, which is central to their reproductive behavior. We sought to determine whether acoustic cues regulate cell proliferation in the brain of adult green treefrogs (Hyla cinerea). Our results show that both male and female treefrogs that heard a conspecific chorus during the breeding season exhibited increased brain cell proliferation compared to animals that heard random tones. Increased cell proliferation, as assessed by the number of 5-bromo-2'-deoxyuridine-immunoreactive (BrdU+) cells, were found near the ventricles of acoustically sensitive brain regions such as the preoptic area (POA) and the infundibular hypothalamus (IF). Sex differences emerged in the location of this socially modulated cell proliferation: increases occurred primarily in the male POA and the female IF. In addition, gonadal steroid hormones might have played a role in the social modulation of cell proliferation: by statistically control- ling for hormone level, we revealed that androgens might influence socially induced increases in BrdU+ cells in the male POA, but estrogen did not contribute to socially induced increases in the female IF. These results indicate that the reception of social cues increases cell proliferation in brain regions mediating sexual behavior and endocrine regulation, and moreover that social modulation of cell proliferation occurs in a sexually differentiated fashion.

Neurosteroid production in the songbird brain: a re-evaluation of core principles

Concepts of brain-steroid signaling have traditionally placed emphasis on the gonads and adrenals as the source of steroids, the strict dichotomy of early developmental ("organizational") and mature ("activational") effects, and a relatively slow mechanism of signaling through intranuclear receptors. Continuing research shows that these concepts are not inaccurate, but they are certainly incomplete. In this review, we focus on the song control circuit of songbird species to demonstrate how each of these concepts is limited. We discuss the solid evidence for steroid synthesis within the brain ("neurosteroidogenesis"), the role of neurosteroids in organizational events that occur both early in development and later in life, and how neurosteroids can act in acute and non-traditional ways. The songbird model therefore illustrates how neurosteroids can dramatically increase the diversity of steroid-sensitive brain functions in a behaviorally-relevant system. We hope this inspires further research and thought into neurosteroid signaling in songbirds and other animals.

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

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

Photoperiodic induced changes in reproductive state of border canaries (Serinus canaria) are associated with marked variation in hypothalamic gonadotropin-releasing hormone immunoreactivity and the volume of song control regions

In temperate zone songbirds, such as canaries (Serinus canaria), seasonal variation in gonadal activity and behavior are associated with marked brain changes. These include gonadotropin-releasing hormone (GnRH) expression and the volume of brain areas controlling song production. Questions have been raised about the consistency of seasonal brain changes in canaries. Laboratory studies of the American singer strain raised doubts as to whether this strain exhibits a robust photoperiodic response along with changes in brain GnRH content, and studies of free-living canaries have failed to identify seasonal changes in volume of song control nuclei. We assessed differences in brain GnRH and the song control system associated with photoperiod-induced variation in reproductive state in Border canaries. We found that males and females maintained for 10 weeks on long days (14L:10D) regress their gonads, exhibit a decline in testosterone and initiate molt; a response consistent with the onset of absolute photorefractoriness (i.e., failed to respond to previously stimulating daylengths). All birds regained photosensitivity (i.e., exhibited gonadal response to stimulating daylengths) after experiencing short days (8L:16D) for 6 weeks. Furthermore, comparisons of birds in either a photosensitive, photostimulated, or photorefractory state revealed a marked increase in GnRH protein expression in the photosensitive and photostimulated birds over photorefractory birds. A similar variation was observed in the volume of key forebrain song nuclei. Thus, Border canaries demonstrate measurable neuroplasticity in response to photoperiodic manipulations. These data, along with previous work on other strains of canaries, indicate the presence of intra-specific variation in photoperiodically regulated neuroplasticity.

Doublecortin as a marker of adult neuroplasticity in the canary song control nucleus HVC

It is established that in songbirds the size of several brain song control nuclei varies seasonally, based on changes in cell size, dendritic branching and, in nucleus HVC, the incorporation of newborn neurons. In the developing and adult mammalian brain, the protein doublecortin (DCX) is expressed in postmitotic neurons and, as a part of the microtubule machinery, required for neuronal migration. We recently showed that in adult canaries, DCX-immunoreactive (ir) cells are present throughout the telencephalon, but the link between DCX and the active neurogenesis observed in songbirds remained uncertain. We demonstrate here that DCX labels recently born cells in the canary telencephalon and that, in parallel with changes in HVC volume, the number of DCX-ir cells is increased specifically in the HVC of testosterone-treated males compared with castrates, and in castrated testosterone-treated males paired with a female as compared with males paired with another male. The numbers of elongated DCX-ir cells (presumptive migrating neurons) and round multipolar DCX-ir cells (differentiating neurons) were also affected by the sex of the subjects and their photoperiodic condition (photosensitive vs photostimulated vs photorefractory). Thus, in canaries the endocrine state, as well as the social or photoperiodic condition independently of variation in steroid hormone action, affects the number of cells expressing a protein involved in neuronal migration specifically in brain areas that incorporate new neurons in the telencephalon. The DCX gene may be one of the targets by which testosterone and social stimuli induce seasonal changes in the volume of song nuclei.

The relationship between nature of social change, age, and position of new neurons and their survival in adult zebra finch brain

Some kinds of neurons are spontaneously recruited in the intact, healthy adult brain, but the variables that affect their survival are not always clear. We show that in caudal nidopallium of adult male zebra finches, the rostrocaudal position of newly recruited neurons, their age (1 vs 3 months), and the nature of social change (complex vs simple) after the neurons were born affect their survival. Greater social complexity promoted the survival of younger new neurons, and the demise of older ones; a less marked social change promoted the survival of older new neurons. These effects were position dependent. We suggest that functional correlations between new neuron recruitment/survival and its inferred benefit to the animal might be better perceived when taking into account the position of cells, their age at the time of life style changes, and the nature and magnitude of the life style change.

Nest of origin predicts adult neuron addition rates in the vocal control system of the zebra finch

Neurogenesis and neuronal replacement in adulthood represent dramatic forms of plasticity that might serve as a substrate for behavioral flexibility. In songbirds, neurons are continually replaced in HVC (used as a proper name), a pre-motor region necessary for the production of learned vocalizations. There are large individual differences in HVC neuron addition. Some of this variation is probably due to individual differences in adult experience; however, it is also possible that heritability or experience early in development constrains the levels of adult neuron addition. As a step toward addressing the latter two possibilities, we explored the extent to which nest of origin predicts rates of HVC neuron addition in adult male zebra finches. One month after injections of [(3)H]-thymidine to mark dividing cells, neuron addition in HVC was found to co-vary among birds that had been nest mates, even when they were housed in different cages as adults. We also tested whether nest mate co-variation might be due to shared adult auditory experience by measuring neuron addition in nest mate pairs after one member was deafened. There were significant differences in neuron addition between hearing and deaf birds but nest mate relationships persisted. These results suggest that variation in genotype and/or early pre- or postnatal experience can account for a large fraction of adult variation in rates of neuron addition. These results also suggest that a major constraint on neurogenesis and the capacity to adjust rates of neuron addition in response to adult auditory experience is established early in development.

Adult neurogenesis and systemic adaptation: animal experiments and clinical perspectives for PTSD

The life-long persistence of neuron production in the adult mammalian central nervous system was established at the end of the 20th century and since then, intensive studies have been carried out to determine the biological role of neuronal turnover in the mature brain. To date, evidence has been found of involvement in learning/memory function and stress-related mental disorders. With a discussion of speculative link between impaired amygdala-relevant neurogenesis and PTSD in an animal model, we here review across species the functional significance of adult neurogenesis from the point of view of systemic adaptation.

Even neural stem cells get the blues: evidence for a molecular link between modulation of adult neurogenesis and depression

An emerging hypothesis, linking modulation of neurogenesis with the onset and subsequent treatment of depression, has received much attention recently as an attractive explanation for successful behavioral changes induced by antidepressant medication in both humans and animals. However, evidence for such a link remains elusive and inconsistent. This review discusses evidence for modulation of neurogenesis as a neurobiological substrate for depression within the context of heterogeneous animal models of depression. Examining the evidence currently available linking neurogenesis and depression is problematic for at least four reasons: 1) approaches to document ongoing neurogenesis and neuronal lineage commitment are varied, making cross-study comparison difficult; 2) as the functional contribution of adult neurogenesis has yet to be completely determined, it is speculative to state a functional significance to changes in neurogenesis; 3) there is diversity in animal models of depression with variable degrees of correlation with human depression; and 4) there remains insufficient knowledge of molecular factors and changes in gene expression that conclusively link neurogenesis modulation and depression. This review examines the current state of evidence regarding the following: 1) consistent data collection delineating the existence of neurogenesis, its stages of progression, and stage modulation; 2) the functional contribution of adult hippocampal neurogenesis and the use of stress-based animal models for its modulation, 3) possible molecular links between antidepressant medication and neurogenesis, specifically neurotrophins and trophic factors; and finally 4) specific suggestions for further investigations necessary to warrant full acceptance of a link between modulation of neurogenesis and depression.

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.

The effect of social environment on singing behavior in the zebra finch (Taeniopygia guttata) and its implication for neuronal recruitment

Previous studies found that complex social environment increases new neuronal recruitment in brains of adult male zebra finches, in comparison with exposure to a simple social environment. These experiments could not determine, however, whether this increase was due to greater amounts of auditory input (amount of auditory information the male is exposed to), or auditory output (amount of song it produces). To answer this question, we raised male zebra finches to adulthood in a controlled environment, and were then exposed them to either a single unfamiliar female (simple social environment) or to 45 unfamiliar zebra finches of both sexes (complex social environment). Their singing behavior was monitored in these new social environments. Birds which were exposed to a simple social environment sang significantly more than birds which were exposed to a complex social environment. This supports the hypothesis that increased neuronal recruitment in birds exposed to a complex social environment correlates with processing and storing of auditory input, and not with song produced by the bird.

Plasticity of the Rufous-winged Sparrow, Aimophila carpalis, song control regions during the monsoon-associated summer breeding period

In most temperate zone songbirds, exposure to increasing photoperiod in the spring stimulates the reproductive system and induces reproductive behaviors. Additionally, the brain regions that control singing (song control regions; SCRs) are larger during the breeding season, thus paralleling changes in the activity of the reproductive system. However, in some birds, environmental factors other than photoperiod initiate breeding. For example, free-living male Rufous-winged Sparrows develop their testes in March due to increasing photoperiod, but have relatively low plasma T until after they begin to breed, usually in July, during the monsoon period when day length is declining. We tested the hypothesis that SCRs grow and singing behavior increases after the monsoon rains begin. We captured adult male Rufous-winged Sparrows in July 2002, 7 days before and 20 days after the monsoon rains began, euthanized birds in the field, collected their brains, and measured SCR volumes from sections immunostained for the neuronal marker NeuN. In June and July 2006, we measured song rates in the field before and after the monsoon rains. SCR volumes were larger and singing behavior increased after the onset of the monsoon rains, coinciding with the initiation of breeding. Unlike in other species studied so far, SCR volumes grew as day length was decreasing. Comparative studies utilizing species that do not breed when day length is increasing may provide information on the relative contributions of various environmental factors to SCR neuroplasticity.

Auditory feedback and song production do not regulate seasonal growth of song control circuits in adult white-crowned sparrows

An important area of research in neuroscience is understanding what properties of brain structure and function are stimulated by sensory experience and behavioral performance. We tested the roles of experience and behavior in seasonal plasticity of the neural circuits that regulate learned song behavior in adult songbirds. Neurons in these circuits receive auditory input and show selective auditory responses to conspecific song. We asked whether auditory input or song production contribute to seasonal growth of telencephalic song nuclei. Adult male Gambel's white-crowned sparrows were surgically deafened, which eliminates auditory input and greatly reduces song production. These birds were then exposed to photoperiod and hormonal conditions that regulate the growth of song nuclei. We measured the volumes of the nuclei HVC, robust nucleus of arcopallium (RA), and area X at 7 and 30 d after exposure to long days plus testosterone in deafened and normally hearing birds. We also assessed song production and examined protein kinase C (PKC) expression because previous research reported that immunostaining for PKC increases transiently after deafening. Deafening did not delay or block the growth of the song nuclei to their full breeding-condition size. PKC activity in RA was not altered by deafening in the sparrows. Song continued to be well structured for up to 10 months after deafening, but song production decreased almost eightfold. These results suggest that neither auditory input nor high rates of song production are necessary for seasonal growth of the adult song control system in this species.

Characterization of CaMKII-expressing neurons within a striatal region implicated in avian vocal learning

In songbirds, an anterior forebrain pathway has been implicated in vocal learning. Within Area X, the striatal-pallidal component of this forebrain pathway, early social tutoring dramatically increases the autophosphorylation of CaMKII (calcium-calmodulin-dependent protein kinase II). Activation of CaMKII often is associated with forms of synaptic plasticity (e.g. LTP) underlying learning and memory, and electrophysiological studies have demonstrated NMDA and dopamine (DA) receptor-dependent LTP among Area X medium spiny neurons [Ding, L., Perkel, D.J., 2002. Dopamine modulates excitability of spiny neurons in the avian Basal Ganglia. J. Neurosci. 22, 5210-5218]. Together, these data suggest that Area X neurons may help to encode a representation of song used for vocal mimicry. To identify which Area X neurons could participate in the CaMKII response to song tutoring, we used immunocytochemistry to assess the colocalization of CaMKII with several other biochemical markers that identify specific neuron classes within Area X. Virtually all (approximately 98%) Area X cells expressing CaMKII also expressed DARPP-32 (dopamine- and adenosine 3'5'-monophosphate-regulated phosphoprotein), a dopamine signaling protein enriched in medium spiny striatal neurons. The implication that medium spiny neurons are primary mediators of the pCaMKII response to tutoring is interesting in view of the established dopaminergic modulation of LTP in this cell type. Additionally, BrdU and DARPP-32 immunocytochemistry were combined to test whether medium spiny neurons are among the neurons generated and incorporated into Area X during song learning. Based upon their expression of DARPP-32, the majority of Area X neurons labeled by BrdU injections given on posthatch days 20-25 are medium spiny neurons.

Genomic resources for songbird research and their use in characterizing gene expression during brain development

Vocal learning and neuronal replacement have been studied extensively in songbirds, but until recently, few molecular and genomic tools for songbird research existed. Here we describe new molecular/genomic resources developed in our laboratory. We made cDNA libraries from zebra finch (Taeniopygia guttata) brains at different developmental stages. A total of 11,000 cDNA clones from these libraries, representing 5,866 unique gene transcripts, were randomly picked and sequenced from the 3' ends. A web-based database was established for clone tracking, sequence analysis, and functional annotations. Our cDNA libraries were not normalized. Sequencing ESTs without normalization produced many developmental stage-specific sequences, yielding insights into patterns of gene expression at different stages of brain development. In particular, the cDNA library made from brains at posthatching day 30-50, corresponding to the period of rapid song system development and song learning, has the most diverse and richest set of genes expressed. We also identified five microRNAs whose sequences are highly conserved between zebra finch and other species. We printed cDNA microarrays and profiled gene expression in the high vocal center of both adult male zebra finches and canaries (Serinus canaria). Genes differentially expressed in the high vocal center were identified from the microarray hybridization results. Selected genes were validated by in situ hybridization. Networks among the regulated genes were also identified. These resources provide songbird biologists with tools for genome annotation, comparative genomics, and microarray gene expression analysis.

Increasing stereotypy in adult zebra finch song correlates with a declining rate of adult neurogenesis

Adult neurogenesis is often correlated with learning new tasks, suggesting that a function of incorporating new neurons is to permit new memory formation. However, in the zebra finch, neurons are added to the song motor pathway throughout life, long after the initial song motor pattern is acquired by about 3 months of age. To explore this paradox, we examined the relationship between adult song structure and neuron addition using sensitive measures of song acoustic structure. We report that between 4 and 15 months of age there was an increase in the stereotypy of fine-grained spectral and temporal features of syllable acoustic structure. These results indicate that the zebra finch continues to refine motor output, perhaps by practice, over a protracted period beyond the time when song is first learned. Over the same age range, there was a decrease in the addition of new neurons to HVC, a region necessary for song production, but not to Area X or the hippocampus, regions not essential for singing. We propose that age-related changes in the stereotypy of syllable acoustic structure and HVC neuron addition are functionally related.

LMAN lesions prevent song degradation after deafening without reducing HVC neuron addition

In some songbirds perturbing auditory feedback can promote changes in song structure well beyond the end of song learning. One factor that may drive vocal change in such deafened birds is the ongoing addition of new vocal-motor neurons into the song system. Without auditory feedback to guide their incorporation, the addition of these new neurons could disrupt the established song pattern. To assess this hypothesis, the authors determined if neuronal recruitment into the vocal motor nucleus HVC is affected by neural signals that influence vocal change in adult deafened birds. Such signals appear to be conveyed via LMAN, a nucleus in the anterior forebrain that is necessary for vocal change after deafening. Here the authors tested whether LMAN lesions might restrict song degradation after deafening by reducing the addition or survival of new HVC neurons that would otherwise corrupt the ongoing song pattern. Using [3H]thymidine autoradiography to identify neurons generated in adult zebra finches, it was shown here that LMAN lesions do not reduce the number or percent of new HVC neurons surviving for either several weeks or months after [3H]thymidine labeling. However, the authors confirmed previous reports that LMAN lesions restrict vocal change after deafening. These data suggest that neurons incorporated into the adult HVC may form behaviorally adaptive connections without requiring auditory feedback, and that any role such neurons may play in promoting vocal change after adult deafening requires anterior forebrain pathway output.

FnTm2, a novel brain-specific transcript, is dynamically expressed in the song learning circuit of the zebra finch

Zebra finch males learn their song by imitation, a process influenced by social variables. The neural pathways for acquisition and production of learned song are known, but the cellular and molecular underpinnings are not. Here we describe a novel gene named "FnTm2" ("Phantom 2") that is predicted to encode a small protein (220 aa) with a single fibronectin type III domain and a single transmembrane domain. This gene shows great variability in its expression in song system neurons of the anterior forebrain pathway (AFP), a circuit that influences song discrimination and is necessary for normal song acquisition. AFP nuclei that express FnTm2 include the nucleus HVC (its Area X-projecting neurons), Area X, and LMAN (core and shell). FnTm2 expression does not correlate with singing behavior like the immediate early gene ZENK. It is expressed variably during sleeping hours and is not dependent on an intact song circuit. FnTm2's expression is sensitive to hearing, because in deafened birds its expression is substantially reduced in the core of LMAN. Furthermore, a comparison of FnTm2 expression between mice and zebra finches revealed a conserved pattern of expression in the "limbic system." We suggest that FnTm2 may be sensitive to affective and/or attentional states and thus may provide insights on how social variables influence the production and discrimination of learned vocalizations.

Hormonal and environmental control of song control region growth and new neuron addition in adult male house finches,Carpodacus mexicanus

In songbirds, testosterone (T) mediates seasonal changes in the sizes and neuroanatomical characteristics of brain regions that control singing (song control regions; SCRs). One model explaining the mechanisms of the growth of one SCR, the HVC, postulates that in the spring increasing photoperiod and circulating T concentrations enhance new neuron survival, thus increasing total neuron number. However, most research investigating the effects of T on new neuron survival has been done in autumn. The present study investigated the effects of photoperiod and T treatment on SCR growth and new neuron survival in the HVC in photosensitive adult male House Finches, Carpodacus mexicanus, under simulated spring-like conditions. Birds were castrated, given T-filled or empty Silastic capsules and maintained on short days (SD; 8L:16D) or long days (LD; 16L:8D). To mark new cells, birds received bromodeoxyuridine injections 11 days after experimental manipulations began and were sacrificed 28 days later. Testosterone treatment increased the sizes of two SCRs, the HVC and Robust nucleus of the arcopallium (RA). Exposure to LD did not affect HVC volume, but did increase RA volume. Testosterone treatment increased the total number of HVC neurons, but did not affect the number of new HVC neurons. Thus, T initiates SCR growth and increases neuron survival, but effects of T on new neuron incorporation may be limited in photosensitive birds under spring-like conditions. These results provide new insight into the effects of photoperiod and T treatment on vernal SCR growth and new neuron incorporation and support current models explaining this growth.