Area-specific migration and recruitment of new neurons in the adult songbird brain

Song system neuroanatomy, and immediate early gene expression in a finch species with extensive male and female song

Birdsong is a relatively well-studied behavior, both due to its importance as a model for vocal production learning and as an intriguing complex social behavior. Until the last few decades, work on birdsong focused almost exclusively on males. However, it is now widely accepted that female song not only exists, but is fairly common throughout the oscine passerines. Despite this, and the large number of researchers who have begun exploring female song in the field, researchers in the lab have been slow to adopt model species with female song. Studying female song in the lab is critical for our understanding of sex-specific factors in the physiology controlling this fascinating behavior. Additionally, as a model for vocal production learning in humans, understanding the mechanistic and neuroendocrine control of female song is clearly important. In this study, we examined the red-cheeked cordon bleu (RCCB), an Estrildid finch species with extensive female song. Specifically, we found that there were no significant sex differences in circulating levels of testosterone and progesterone, nor in song production rate. There were no significant differences in cell densities in the three nuclei of the song control system we examined. Additionally, the volume of the robust nucleus of the arcopallium was not significantly different and we report the smallest sex difference in HVC yet published in a songbird. Finally, we demonstrated similar levels of motor driven immediate early gene expression in both males and females after song production.

Differences in vocal brain areas and astrocytes between the house wren and the rufous-tailed hummingbird

The house wren shows complex song, and the rufous-tailed hummingbird has a simple song. The location of vocal brain areas supports the song's complexity; however, these still need to be studied. The astrocytic population in songbirds appears to be associated with change in vocal control nuclei; however, astrocytic distribution and morphology have not been described in these species. Consequently, we compared the distribution and volume of the vocal brain areas: HVC, RA, Area X, and LMAN, cell density, and the morphology of astrocytes in the house wren and the rufous-tailed hummingbird. Individuals of the two species were collected, and their brains were analyzed using serial Nissl- NeuN- and MAP2-stained tissue scanner imaging, followed by 3D reconstructions of the vocal areas; and GFAP and S100β astrocytes were analyzed in both species. We found that vocal areas were located close to the cerebral midline in the house wren and a more lateralized position in the rufous-tailed hummingbird. The LMAN occupied a larger volume in the rufous-tailed hummingbird, while the RA and HVC were larger in the house wren. While Area X showed higher cell density in the house wren than the rufous-tailed hummingbird, the LMAN showed a higher density in the rufous-tailed hummingbird. In the house wren, GFAP astrocytes in the same bregma where the vocal areas were located were observed at the laminar edge of the pallium (LEP) and in the vascular region, as well as in vocal motor relay regions in the pallidum and mesencephalon. In contrast, GFAP astrocytes were found in LEP, but not in the pallidum and mesencephalon in hummingbirds. Finally, when comparing GFAP astrocytes in the LEP region of both species, house wren astrocytes exhibited significantly more complex morphology than those of the rufous-tailed hummingbird. These findings suggest a difference in the location and cellular density of vocal circuits, as well as morphology of GFAP astrocytes between the house wren and the rufous-tailed hummingbird.

In vivo imaging in transgenic songbirds reveals superdiffusive neuron migration in the adult brain

Neuron migration is a key phase of neurogenesis, critical for the assembly and function of neuronal circuits. In songbirds, this process continues throughout life, but how these newborn neurons disperse through the adult brain is unclear. We address this question using in vivo two-photon imaging in transgenic zebra finches that express GFP in young neurons and other cell types. In juvenile and adult birds, migratory cells are present at a high density, travel in all directions, and make frequent course changes. Notably, these dynamic migration patterns are well fit by a superdiffusive model. Simulations reveal that these superdiffusive dynamics are sufficient to disperse new neurons throughout the song nucleus HVC. These results suggest that superdiffusive migration may underlie the formation and maintenance of nuclear brain structures in the postnatal brain and indicate that transgenic songbirds are a useful resource for future studies into the mechanisms of adult neurogenesis.

Neuroestradiol and neuronal development: Not an exclusive male tale anymore

The brain synthesizes a variety of neurosteroids, including neuroestradiol. Inhibition of neuroestradiol synthesis results in alterations in basic neurodevelopmental processes, such as neurogenesis, neuroblast migration, neuritogenesis and synaptogenesis. Although the neurodevelopmental actions of neuroestradiol are exerted in both sexes, some of them are sex-specific, such as the well characterized effects of neuroestradiol derived from the metabolism of testicular testosterone during critical periods of male brain development. In addition, recent findings have shown sex-specific actions of neuroestradiol on neuroblast migration, neuritic growth and synaptogenesis in females. Among other factors, the epigenetic regulation exerted by X linked genes, such as Kdm6a/Utx, may determine sex-specific actions of neuroestradiol in the female brain. This review evidences the impact of neuroestradiol on brain formation in both sexes and highlights the interaction of neural steriodogenesis, hormones and sex chromosomes in sex-specific brain development.

Steroid-dependent plasticity in the song control system: Perineuronal nets and HVC neurogenesis

The vocal control nucleus HVC in songbirds has emerged as a widespread model system to study adult brain plasticity in response to changes in the hormonal and social environment. I review here studies completed in my laboratory during the last decade that concern two aspects of this plasticity: changes in aggregations of extracellular matrix components surrounding the soma of inhibitory parvalbumin-positive neurons called perineuronal nets (PNN) and the production/incorporation of new neurons. Both features are modulated by the season, age, sex and endocrine status of the birds in correlation with changes in song structure and stability. Causal studies have also investigated the role of PNN and of new neurons in the control of song. Dissolving PNN with chondroitinase sulfate, a specific enzyme applied directly on HVC or depletion of new neurons by focalized X-ray irradiation both affected song structure but the amplitude of changes was limited and deserves further investigations.

Notch1 is involved in cell proliferation and neuronal differentiation in the HVC of zebra finch (Taeniopygia guttata)

Significant sex differences are found in songbirds' song control nuclei and their controlled song behaviors. To elucidate the underlying mechanisms, we explored the role of Notch1 during the development of the high vocal centre (HVC) and song learning in zebra finch. Our study first found that Notch1 positive cells were distributed in HVC with female-biased densities at posthatching day (PHD) 15, but male-biased at PHD 45 and adult. There were about 60 putative oestrogen-responsive elements within 2.5 kb upstream of Notch1, and Notch1 mRNA in the explants that contained the developing male HVC was significantly increased after estrogen addition into the cultured medium for 48 h. After injecting Notch1-interfering lentivirus into the male or female HVC at PHD 15, cell proliferation was significantly promoted in the ventricle zone overlying the HVC at PHD 23. In addition, neuronal differentiation towards Hu+ /BrdU+ at PHD 31, mature neurons (NeuN+/BrdU+) including those projecting to RA in HVC and the sizes of HVC and RA at adult increased significantly after Notch1-interfering lentiviruses were injected into the male HVC at PHD 15. However, the above measurements decreased, following the injection of the lentiviruses expressing Notch intracellular domain (NICD). Finally, the repeat numbers of syllables 'b' or 'c' of learned songs changed after the injection of Notch1-interfering or NICD-expressing lentiviruses into the HVC at PHD15. Our study suggests that Notch1 is related to the development of HVC and song learning in the zebra finch.

Effects of the depletion of neural progenitors by focal X-ray irradiation on song production and perception in canaries

The song control nucleus HVC of songbirds has emerged as a widespread model system to study adult neurogenesis and the factors that modulate the incorporation of new neurons, including seasonal state, sex differences or sex steroid hormone concentrations. However, the specific function of these new neurons born in adulthood remains poorly understood. We implemented a new procedure based on focal X-ray irradiation to deplete neural progenitors in the ventricular zone adjacent to HVC and study the functional consequences. A 23 Gy dose depleted by more than 50 percent the incorporation of BrdU in neural progenitors, a depletion that was confirmed by a significant decrease in doublecortin positive neurons. This depletion of neurogenesis significantly increased the variability of testosterone-induced songs in females and decreased their bandwidth. Expression of the immediate early gene ZENK in secondary auditory areas of the telencephalon that respond to song was also inhibited. These data provide evidence that new neurons in HVC play a role in both song production and perception and that X-ray focal irradiation represents an excellent tool to advance our understanding of adult neurogenesis.

Inhibition of miR-128 Enhances Vocal Sequence Organization in Juvenile Songbirds

The molecular mechanisms underlying learned vocal communication are not well characterized. This is a major barrier for developing treatments for conditions affecting social communication, such as autism spectrum disorder (ASD). Our group previously generated an activity-dependent gene expression network in the striatopallidal song control nucleus, Area X, in adult zebra finches to identify master regulators of learned vocal behavior. This dataset revealed that the two host genes for microRNA-128, ARPP21 and R3HDM1, are among the top genes whose expression correlates to how much birds sing. Here we examined whether miR-128 itself is behaviorally regulated in Area X and found that its levels decline with singing. We hypothesized that reducing miR-128 during the critical period for vocal plasticity would enhance vocal learning. To test this, we bilaterally injected an antisense miR-128 construct (AS miR-128) or a control scrambled sequence into Area X at post-hatch day 30 (30 d) using sibling-matched experimental and control pupils. The juveniles were then returned to their home cage and raised with their tutors. Strikingly, inhibition of miR-128 in young birds enhanced the organization of learned vocal sequences. Tutor and pupil stereotypy scores were positively correlated, though the correlation was stronger between tutors and control pupils compared to tutors and AS miR-128 pupils. This difference was driven by AS miR-128 pupils achieving higher stereotypy scores despite their tutors’ lower syntax scores. AS miR-128 birds with tutors on the higher end of the stereotypy spectrum were more likely to produce songs with faster tempos relative to sibling controls. Our results suggest that low levels of miR-128 facilitate vocal sequence stereotypy. By analogy, reducing miR-128 could enhance the capacity to learn to speak in patients with non-verbal ASD. To our knowledge, this study is the first to directly link miR-128 to learned vocal communication and provides support for miR-128 as a potential therapeutic target for ASD.

Neurogenesis and the development of neural sex differences in vocal control regions of songbirds

The brain regions that control the learning and production of song and other learned vocalizations in songbirds exhibit some of the largest sex differences in the brain known in vertebrates and are associated with sex differences in singing behavior. Song learning takes place through multiple stages: an early sensory phase when song models are memorized, followed by a sensorimotor phase in which auditory feedback is used to modify song output through subsong, plastic song, to adult crystalized song. However, how patterns of neurogenesis in these brain regions change through these learning stages, and differ between the sexes, is little explored. We collected brains from 63 young male and female zebra finches (Taeniopygia guttata) over four stages of song learning. Using neurogenesis markers for cell division (proliferating cell nuclear antigen), neuron migration (doublecortin), and mature neurons (neuron-specific nuclear protein), we demonstrate that there are sex-specific changes in neurogenesis over song development that differ between the caudal motor pathway and anterior forebrain pathway of the vocal control circuit. In many of these regions, sex differences emerged very early in development, by 25 days post hatch, at the beginning of song learning. The emergence of sex differences in other components of the system was more gradual and had specific trajectories depending on the brain region and its function. In conclusion, we found that sex differences occurred early and continued during song learning. Moreover, transitions from the different phases of song development do not seem to depend on large changes in neurogenesis in the vocal control areas measured.

Evidence That Artificial Light at Night Induces Structure-Specific Changes in Brain Plasticity in a Diurnal Bird

We recently reported that artificial light at night (ALAN), at ecologically relevant intensities (1.5, 5 lux), increases cell proliferation in the ventricular zone and recruitment of new neurons in several forebrain regions of female zebra finches (Taeniopygia guttata), along with a decrease of total neuronal densities in some of these regions (indicating possible neuronal death). In the present study, we exposed male zebra finches to the same ALAN intensities, treated them with 5′-bromo-2′-deoxyuridine, quantified cell proliferation and neuronal recruitment in several forebrain regions, and compared them to controls that were kept under dark nights. ALAN increased cell proliferation in the ventricular zone, similar to our previous findings in females. We also found, for the first time, that ALAN increased new neuronal recruitment in HVC and Area X, which are part of the song system in the brain and are male-specific. In other brain regions, such as the medial striatum, nidopallium caudale, and hippocampus, we recorded an increased neuronal recruitment only in the medial striatum (unlike our previous findings in females), and relative to the controls this increase was less prominent than in females. Moreover, the effect of ALAN duration on total neuronal densities in the studied regions varied between the sexes, supporting the suggestion that males are more resilient to ALAN than females. Suppression of nocturnal melatonin levels after ALAN exhibited a light intensity-dependent decrease in males in contrast to females, another indication that males might be less affected by ALAN. Taken together, our study emphasizes the importance of studying both sexes when considering ALAN effects on brain plasticity.

Effectivity of Two Cell Proliferation Markers in Brain of a Songbird Zebra Finch

Simple Summary

The present study compared the effectivity of two cell proliferation markers, BrdU and EdU, in the brain neurogenic zone of the songbird zebra finch. It shows their saturation doses, that BrdU labels more cells than the equimolar dose of EdU, and that both markers can be reliably detected in the same brain.

Abstract

There are two most heavily used markers of cell proliferation, thymidine analogues 5-bromo-2′-deoxyuridine (BrdU) and 5-ethynyl-2′-deoxyuridine (EdU) that are incorporated into the DNA during its synthesis. In neurosciences, they are often used consecutively in the same animal to detect neuronal populations arising at multiple time points, their migration and incorporation. The effectivity of these markers, however, is not well established. Here, we studied the effectivity of equimolar doses of BrdU and EdU to label new cells and looked for the dose that will label the highest number of proliferating cells in the neurogenic ventricular zone (VZ) of adult songbirds. We found that, in male zebra finches (Taeniopygia guttata), the equimolar doses of BrdU and EdU did not label the same number of cells, with BrdU being more effective than EdU. Similarly, in liver, BrdU was more effective. The saturation of the detected brain cells occurred at 50 mg/kg BrdU and above 41 mg/kg EdU. Higher dose of 225 mg/kg BrdU or the equimolar dose of EdU did not result in any further significant increases. These results show that both markers are reliable for the detection of proliferating cells in birds, but the numbers obtained with BrdU and EdU should not be compared.

Putative adult neurogenesis in palaeognathous birds: The common ostrich (Struthio camelus) and emu (Dromaius novaehollandiae)

In the current study, we examined adult neurogenesis throughout the brain of the common ostrich (Struthio camelus) and emu (Dromaius novaehollandiae) using immunohistochemistry for the endogenous markers PCNA which labels proliferating cells, and DCX, which stains immature and migrating neurons. The distribution of PCNA and DCX labelled cells was widespread throughout the brain of both species. The highest density of cells immunoreactive to both markers was observed in the olfactory bulbs and the telencephalon, especially the subventricular zone of the lateral ventricle. Proliferative hot spots, identified with strong PCNA and DCX immunolabelling, were identified in the dorsal and ventral poles of the rostral aspects of the lateral ventricles. The density of PCNA immunoreactive cells was less in the telencephalon of the emu compared to the common ostrich. Substantial numbers of PCNA immunoreactive cells were observed in the diencephalon and brainstem, but DCX immunoreactivity was weaker in these regions, preferentially staining axons and dendrites over cell bodies, except in the medial regions of the hypothalamus where distinct DCX immunoreactive cells and fibres were observed. PCNA and DCX immunoreactive cells were readily observed in moderate density in the cortical layers of the cerebellum of both species. The distribution of putative proliferating cells and immature neurons in the brain of the common ostrich and the emu is widespread, far more so than in mammals, and compares with the neognathous birds, and suggests that brain plasticity and neuronal turnover is an important aspect of cognitive brain functions in these birds.

Cell death in the avian brain with emphasis on the development and plasticity of the song control system

Programmed cell death is a fundamental feature of brain development, homeostasis, and adult plasticity. One model system, in which the role of cell death in establishment, maintenance and plasticity of neural tissues is evident throughout both early development and in the adult, is the neural circuitry underlying the learning and production of singing behavior in songbirds. The dramatic sexual dimorphism and natural, cyclical growth and regression of the song control system provides a useful environment for studying programmed cell death. Especially valuable and unique to songbirds, the occurrence of cell death in the song control system is correlated to quantifiable changes in a biologically relevant and learned sensorimotor behavior-that is singing. Within this review I explore the topic of cell death in the avian brain primarily within the context of the song circuits. I first establish why songbirds are a useful model for studying cell death and provide a brief overview of the organization of the circuitry underlying song learning and production. I then discuss the processes and mechanisms of cell death during early development and sexual differentiation of the song control system. I present the classic and recent work exploring cell death in the adult avian brain by covering topics of homeostasis and neuronal turnover, seasonal plasticity, and neural injury and insult. Finally, I propose several outstanding questions in the field of cell death biology in the avian brain, which when addressed have great potential to provide unique insight into the role of cell death in the organization and maintenance of neural tissues, the plasticity of developmentally organized neural circuits in the adult, and the mechanisms underlying functional recovery from both natural and injury-induced neurodegeneration.

Development of vernal migration in redheaded buntings: concurrent behavioral, physiological and neural changes under stimulatory photoperiods

We investigated changes in behavior, physiology and selected brain regions during the development of vernal migration and reproduction phenotypes in migratory redheaded buntings.

Decreased Neurogenesis Increases Spatial Reversal Errors in Chickadees (Poecile atricapillus)

Adult hippocampal neurogenesis has been proposed to both aid memory formation and disrupt memory. We examined the role of adult hippocampal neurogenesis in spatial working and reference memory in black-capped chickadees (Poecile atricapillus), a passerine bird that relies on spatial memory for cache retrieval and foraging. We tested spatial working and spatial reference memory in birds that had received methylazoxymethanol acetate (MAM), a neurotoxin that decreases hippocampal neurogenesis. MAM treatment significantly reduced neurogenesis in the hippocampus quantified by doublecortin (DCX) labeling of newly divided and migrating neurons. MAM treatment had little effect on the working or reference memory but caused an increase in errors on the reference memory task following reversal. Working memory for recently visited spatial locations and reference memory for familiar spatial locations were thus unaffected by a reduction in neurogenesis. An increase in errors following reference memory reversal may indicate that adult hippocampal neurogenesis aids in pattern separation, the differentiation of similar memories at the time of encoding.

Sex Steroids, Adult Neurogenesis, and Inflammation in CNS Homeostasis, Degeneration, and Repair

Sex steroidal hormones coordinate the development and maintenance of tissue architecture in many organs, including the central nervous systems (CNS). Within the CNS, sex steroids regulate the morphology, physiology, and behavior of a wide variety of neural cells including, but not limited to, neurons, glia, endothelial cells, and immune cells. Sex steroids spatially and temporally control distinct molecular networks, that, in turn modulate neural activity, synaptic plasticity, growth factor expression and function, nutrient exchange, cellular proliferation, and apoptosis. Over the last several decades, it has become increasingly evident that sex steroids, often in conjunction with neuroinflammation, have profound impact on the occurrence and severity of neuropsychiatric and neurodegenerative disorders. Here, I review the foundational discoveries that established the regulatory role of sex steroids in the CNS and highlight recent advances toward elucidating the complex interaction between sex steroids, neuroinflammation, and CNS regeneration through adult neurogenesis. The majority of recent work has focused on neuroinflammatory responses following acute physical damage, chronic degeneration, or pharmacological insult. Few studies directly assess the role of immune cells in regulating adult neurogenesis under healthy, homeostatic conditions. As such, I also introduce tractable, non-traditional models for examining the role of neuroimmune cells in natural neuronal turnover, seasonal plasticity of neural circuits, and extreme CNS regeneration.

Putative Adult Neurogenesis in Old World Parrots: The Congo African Grey Parrot (Psittacus erithacus) and Timneh Grey Parrot (Psittacus timneh)

In the current study, we examined for the first time, the potential for adult neurogenesis throughout the brain of the Congo African grey parrot (Psittacus erithacus) and Timneh grey parrot (Psittacus timneh) using immunohistochemistry for the endogenous markers proliferating cell nuclear antigen (PCNA), which labels proliferating cells, and doublecortin (DCX), which stains immature and migrating neurons. A similar distribution of PCNA and DCX immunoreactivity was found throughout the brain of the Congo African grey and Timneh grey parrots, but minor differences were also observed. In both species of parrots, PCNA and DCX immunoreactivity was observed in the olfactory bulbs, subventricular zone of the lateral wall of the lateral ventricle, telencephalic subdivisions of the pallium and subpallium, diencephalon, mesencephalon and the rhombencephalon. The olfactory bulb and telencephalic subdivisions exhibited a higher density of both PCNA and DCX immunoreactive cells than any other brain region. DCX immunoreactive staining was stronger in the telencephalon than in the subtelencephalic structures. There was evidence of proliferative hot spots in the dorsal and ventral poles of the lateral ventricle in the Congo African grey parrots at rostral levels, whereas only the dorsal accumulation of proliferating cells was observed in the Timneh grey parrot. In most pallial regions the density of PCNA and DCX stained cells increased from rostral to caudal levels with the densest staining in the nidopallium caudolaterale (NCL). The widespread distribution of PCNA and DCX in the brains of both parrot species suggest the importance of adult neurogenesis and neuronal plasticity during learning and adaptation to external environmental variations.

Cellular and molecular attributes of neural stem cell niches in adult zebrafish brain

Adult neurogenesis is a complex, presumably conserved phenomenon in vertebrates with a broad range of variations regarding neural progenitor/stem cell niches, cellular composition of these niches, migratory patterns of progenitors and so forth among different species. Current understanding of the reasons underlying the inter-species differences in adult neurogenic potential, the identification and characterization of various neural progenitors, characterization of the permissive environment of neural stem cell niches and other important aspects of adult neurogenesis is insufficient. In the last decade, zebrafish has emerged as a very useful model for addressing these questions. In this review, we have discussed the present knowledge regarding the neural stem cell niches in adult zebrafish brain as well as their cellular and molecular attributes. We have also highlighted their similarities and differences with other vertebrate species. In the end, we shed light on some of the known intrinsic and extrinsic factors that are assumed to regulate the neurogenic process in adult zebrafish brain. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1188-1205, 2017.

Studies of HVC Plasticity in Adult Canaries Reveal Social Effects and Sex Differences as Well as Limitations of Multiple Markers Available to Assess Adult Neurogenesis

In songbirds, neurogenesis in the song control nucleus HVC is sensitive to the hormonal and social environment but the dynamics of this process is difficult to assess with a single exogenous marker of new neurons. We simultaneously used three independent markers to investigate HVC neurogenesis in male and female canaries. Males were castrated, implanted with testosterone and housed either alone (M), with a female (M-F) or with another male (M-M) while females were implanted with 17β-estradiol and housed with a male (F-M). All subjects received injections of the two thymidine analogues, BrdU and of EdU, respectively 21 and 10 days before brain collection. Cells containing BrdU or EdU or expressing doublecortin (DCX), which labels newborn neurons, were quantified. Social context and sex differentially affected total BrdU+, EdU+, BrdU+EdU- and DCX+ populations. M-M males had a higher density of BrdU+ cells in the ventricular zone adjacent to HVC and of EdU+ in HVC than M-F males. M birds had a higher ratio of BrdU+EdU- to EdU+ cells than M-F subjects suggesting higher survival of newer neurons in the former group. Total number of HVC DCX+ cells was lower in M-F than in M-M males. Sex differences were also dependent of the type of marker used. Several technical limitations associated with the use of these multiple markers were also identified. These results indicate that proliferation, recruitment and survival of new neurons can be independently affected by environmental conditions and effects can only be fully discerned through the use of multiple neurogenesis markers.

Putative adult neurogenesis in two domestic pigeon breeds (Columba livia domestica): racing homer versus utility carneau pigeons

Generation of neurons in the brains of adult birds has been studied extensively in the telencephalon of song birds and few studies are reported on the distribution of PCNA and DCX in the telencephalon of adult non-song learning birds. We report here on adult neurogenesis throughout the brains of two breeds of adult domestic pigeons (Columba livia domestica), the racing homer and utility carneau using endogenous immunohistochemical markers proliferating cell nuclear antigen (PCNA) for proliferating cells and doublecortin (DCX) for immature and migrating neurons. The distribution of PCNA and DCX immunoreactivity was very similar in both pigeon breeds with only a few minor differences. In both pigeons, PCNA and DCX immunoreactivity was observed in the olfactory bulbs, walls of the lateral ventricle, telencephalic subdivisions of the pallium and subpallium, diencephalon, mesencephalon and cerebellum. Generally, the olfactory bulbs and telencephalon had more PCNA and DCX cells than other regions. Two proliferative hotspots were evident in the dorsal and ventral poles of the lateral ventricles. PCNA- and DCX-immunoreactive cells migrated radially from the walls of the lateral ventricle into the parenchyma. In most telencephalic regions, the density of PCNA- and DCX-immunoreactive cells increased from rostral to caudal, except in the mesopallium where the density decreased from rostral to middle levels and then increased caudally. DCX immunoreactivity was more intense in fibres than in cell bodies and DCX-immunoreactive cells included small granular cells, fusiform bipolar cells, large round and or polygonal multipolar cells. The similarity in the distribution of proliferating cells and new neurons in the telencephalon of the two breeds of pigeons may suggest that adult neurogenesis is a conserved trait as an ecological adaptation irrespective of body size.

Adult Neurogenesis in the Songbird: Region-Specific Contributions of New Neurons to Behavioral Plasticity and Stability

Our understanding of the role of new neurons in learning and encoding new information has been largely based on studies of new neurons in the mammalian dentate gyrus and olfactory bulb - brain regions that may be specialized for learning. Thus the role of new neurons in regions that serve other functions has yet to be fully explored. The song system provides a model for studying new neuron function in brain regions that contribute differently to song learning, song auditory discrimination, and song motor production. These regions subserve learning as well as long-term storage of previously learned information. This review examines the differences between learning-based and activity-based retention of new neurons and explores the potential contributions of new neurons to behavioral stability in the song motor production pathway.

Age-Dependent Neurogenesis and Neuron Numbers within the Olfactory Bulb and Hippocampus of Homing Pigeons

Many birds are supreme long-distance navigators that develop their navigational ability in the first months after fledgling but update the memorized environmental information needed for navigation also later in life. We studied the extent of juvenile and adult neurogenesis that could provide such age-related plasticity in brain regions known to mediate different mechanisms of pigeon homing: the olfactory bulb (OB), and the triangular area of the hippocampal formation (HP tr). Newly generated neurons (visualized by doublecortin, DCX) and mature neurons were counted stereologically in 35 pigeon brains ranging from 1 to 168 months of age. At the age of 1 month, both areas showed maximal proportions of DCX positive neurons, which rapidly declined during the first year of life. In the OB, the number of DCX-positive periglomerular neurons declined further over time, but the number of mature periglomerular cells appeared unchanged. In the hippocampus, the proportion of DCX-positive neurons showed a similar decline yet to a lesser extent. Remarkably, in the triangular area of the hippocampus, the oldest birds showed nearly twice the number of neurons as compared to young adult pigeons, suggesting that adult born neurons in these regions expanded the local circuitry even in aged birds. This increase might reflect navigational experience and, possibly, expanded spatial memory. On the other hand, the decrease of juvenile neurons in the aging OB without adding new circuitry might be related to the improved attachment to the loft characterizing adult and old pigeons.

Endocrine and social regulation of adult neurogenesis in songbirds

The identification of pronounced seasonal changes in the volume of avian song control nuclei stimulated the discovery of adult neurogenesis in songbirds as well as renewed studies in mammals including humans. Neurogenesis in songbirds is modulated by testosterone and other factors such as photoperiod, singing activity and social environment. Adult neurogenesis has been widely studied by labeling, with tritiated thymidine or its analog BrdU, cells duplicating their DNA in anticipation of their last mitotic division and following their fate as new neurons. New methods based on endogenous markers of cell cycling or of various stages of neuronal life have allowed for additional progress. In particular immunocytochemical visualization of the microtubule-associated protein doublecortin has provided an integrated view of neuronal replacement in the song control nucleus HVC. Multiple questions remain however concerning the specific steps in the neuronal life cycle that are modulated by various factors and the underlying cellular mechanisms.

Network analysis of microRNA and mRNA seasonal dynamics in a highly plastic sensorimotor neural circuit

Background

Adult neurogenesis and the incorporation of adult-born neurons into functional circuits requires precise spatiotemporal coordination across molecular networks regulating a wide array of processes, including cell proliferation, apoptosis, neurotrophin signaling, and electrical activity. MicroRNAs (miRs) - short, non-coding RNA sequences that alter gene expression by post-transcriptional inhibition or degradation of mRNA sequences - may be involved in the global coordination of such diverse biological processes. To test the hypothesis that miRs related to adult neurogenesis and related cellular processes are functionally regulated in the nuclei of the avian song control circuit, we used microarray analyses to quantify changes in expression of miRs and predicted target mRNAs in the telencephalic nuclei HVC, the robust nucleus of arcopallium (RA), and the basal ganglia homologue Area X in breeding and nonbreeding Gambel’s white-crowned sparrows (Zonotrichia leucophrys gambelli).

Results

We identified 46 different miRs that were differentially expressed across seasons in the song nuclei. miR-132 and miR-210 showed the highest differential expression in HVC and Area X, respectively. Analyzing predicted mRNA targets of miR-132 identified 33 candidate target genes that regulate processes including cell cycle control, calcium signaling, and neuregulin signaling in HVC. Likewise, miR-210 was predicted to target 14 mRNAs differentially expressed across seasons that regulate serotonin, GABA, and dopamine receptor signaling and inflammation.

Conclusions

Our results identify potential miR–mRNA regulatory networks related to adult neurogenesis and provide opportunities to discover novel genetic control of the diverse biological processes and factors related to the functional incorporation of new neurons to the adult brain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2175-z) contains supplementary material, which is available to authorized users.

Sex and age differences in brain-derived neurotrophic factor and vimentin in the zebra finch song system: Relationships to newly generated cells

The neural song circuit is enhanced in male compared with female zebra finches due to differential rates of incorporation and survival of cells between the sexes. Two double-label immunohistochemical experiments were conducted to increase the understanding of relationships between newly generated cells (marked with bromodeoxyuridine [BrdU]) and those expressing brain-derived neurotrophic factor (BDNF) and vimentin, a marker for radial glia. The song systems of males and females were investigated at posthatching day 25 during a heightened period of sexual differentiation (following BrdU injections on days 6-10) and in adulthood (following a parallel injection paradigm). In both HVC (proper name) and the robust nucleus of the arcopallium (RA), about half of the BrdU-positive cells expressed BDNF across sexes and ages. Less than 10% of the BDNF-positive cells expressed BrdU, but this percentage was greater in juveniles than adults. Across both brain regions, more BDNF-positive cells were detected in males compared with females. In RA, the number of these cells was also greater in juveniles than adults. In HVC, the average cross-sectional area covered by the vimentin labeling was greater in males than females and in juveniles compared with adults. In RA, more vimentin was detected in juveniles than adults, and within adults it was greater in females. In juveniles only, BrdU-positive cells appeared in contact with vimentin-labeled fibers in HVC, RA, and Area X. Collectively, the results are consistent with roles of BDNF- and vimentin-labeled cells influencing sexually differentiated plasticity of the song circuit.

Neurogenesis in the adult avian song-control system

New neurons are added throughout the forebrain of adult birds. The song-control system is a model to investigate the addition of new long-projection neurons to a cortical circuit that regulates song, a learned sensorimotor behavior. Neuroblasts destined for the song nucleus HVC arise in the walls of the lateral ventricle, and wander through the pallium to reach HVC. The survival of new HVC neurons is supported by gonadally secreted testosterone and its downstream effectors including neurotrophins, vascularization, and electrical activity of postsynaptic neurons in nucleus RA (robust nucleus of the arcopallium). In seasonal species, the HVC→RA circuit degenerates in nonbreeding birds, and is reconstructed by the incorporation of new projection neurons in breeding birds. There is a functional linkage between the death of mature HVC neurons and the birth of new neurons. Various hypotheses for the function of adult neurogenesis in the song system can be proposed, but this remains an open question.

Doublecortin is widely expressed in the developing and adult retina of sharks

Doublecortin (DCX) is a microtubule-associated protein that has been considered a marker for neuronal precursors and young migrating neurons during the development of the central nervous system and in adult neurogenic niches. The retina of fishes represents an accessible, continuously growing and highly structured (layered) part of the central nervous system and, therefore, offers an exceptional model to extend our knowledge on the possible role of DCX in promoting neurogenesis and migration to appropriate layers. We have analyzed the distribution of DCX in the embryonic and postembryonic retina of a small shark, the lesser spotted dogfish Scyliorhinus canicula, by means of immunohistochemistry. We investigated the relationship between DCX expression and the neurogenic state of DCX-labeled cells by exploring its co-localization with the proliferation marker PCNA (proliferating cell nuclear antigen) and the marker of neuronal differentiation HuC/D. Since radially migrating neurons use radial glial fibers as substrate, we explored the possible correlation between DCX expression and cell migration along radial glia by comparing its expression with that of the glial marker GFAP (glial fibrillary acidic protein). Additionally, we characterized DCX-expressing cells by double immunocytochemistry using antibodies against Calbindin (a marker for mature bipolar and horizontal cells in this species) and Pax6, which has been proposed as a regulator of cell proliferation, cell differentiation, and neuron diversification in the neural retina of sharks. Strong DCX immunoreactivity was observed in immature cells and cell processes, at a time when retinal cells were not yet organized into different laminae. DCX was also found in subsets of mature ganglion, amacrine, bipolar and horizontal cells long after they had exited the cell cycle, a pattern that was maintained in juveniles and adults. Our results on DCX expression in the retina are compatible with a role for DCX in cell migration within the immature retina, and in dynamic neuronal plasticity in the mature retina. We also provide evidence of DCX expression in discrete cells in the retinal pigment epithelium of prehatching embryos and juveniles, which suggest that retinal pigmented epithelial cells in sharks, as in mammals, have an intrinsic capacity to proliferate and differentiate into cells with neural identity.

Endogenous versus exogenous markers of adult neurogenesis in canaries and other birds: advantages and disadvantages

Although the existence of newborn neurons had originally been suggested, but not broadly accepted, based on studies in adult rodent brains, the presence of an active neurogenesis process in adult homoeothermic vertebrates was first firmly established in songbirds. Adult neurogenesis was initially studied with the tritiated thymidine technique, later replaced by the injection and detection of the marker of DNA replication 5-bromo-2'-deoxyuridine (BrdU). More recently, various endogenous markers were used to identify young neurons or cycling neuronal progenitors. We review here the respective advantages and pitfalls of these different approaches in birds, with specific reference to the microtubule-associated protein, doublecortin (DCX), that has been extensively used to identify young newly born neurons in adult brains. All these techniques of course have limitations. Exogenous markers label cells replicating their DNA only during a brief period and it is difficult to select injection doses that would exhaustively label all these cells without inducing DNA damage that will also result in some form of labeling during repair. On the other hand, specificity of endogenous markers is difficult to establish due to problems related to the specificity of antibodies (these problems can be, but are not always, addressed) and more importantly because it is difficult, if not impossible, to prove that a given marker exhaustively and specifically labels a given cell population. Despite these potential limitations, these endogenous markers and DCX staining in particular clearly represent a useful approach to the detailed study of neurogenesis especially when combined with other techniques such as BrdU.

Sexual differences in cell proliferation in the ventricular zone, cell migration and differentiation in the HVC of juvenile Bengalese finch

Song control nuclei have distinct sexual differences and thus are an ideal model to address how brain areas are sexually differentiated. Through a combination of histological analysis and electrical lesions, we first identified the ventricle site for HVC progenitor cells. We then found that there were significant sex differences in the cellular proliferation activity in the ventricular zone of the HVC, the number of migrating cells along the radial cells (positive immunoreactions to vimentin) and differentiation towards neurons. Through co-culturing of male and female slices containing the developing HVC in the same well, we found that the male slices could produce diffusible substances to masculinize the female HVC. By adding estrogen, an estrogen antagonist, brain-derived neurotrophic factor (BDNF) or its antibody into the culture medium, separately or in combination, we found that these diffusible substances may include estrogen and BDNF. Finally, we found that 1) estrogen-induced BDNF upregulation could be detected 48 hr after estrogen treatment and could not be blocked by a vascular endothelial growth factor (VEGF) receptor inhibitor and 2) the amount of VEGF mRNA expressed in the developing HVC and its adjacent area did not display any significant sex differences, as did the distribution of VEGF and laminin-expressing endothelial cells in the developing HVC. Because these findings are largely different from previous reports on the adult female HVC, it is suggested that our estrogen-induced BDNF up-regulation and the resultant sexual differentiation might not be mediated by VEGF and endothelial cells, but instead, may result from the direct effects of estrogen on BDNF.

Developmental, tract-tracing and immunohistochemical study of the peripheral olfactory system in a basal vertebrate: insights on Pax6 neurons migrating along the olfactory nerve

The olfactory system represents an excellent model for studying different aspects of the development of the nervous system ranging from neurogenesis to mechanisms of axon growth and guidance. Important findings in this field come from comparative studies. We have analyzed key events in the development of the olfactory system of the shark Scyliorhinus canicula by combining immunohistochemical and tract-tracing methods. We describe for the first time in a cartilaginous fish an early population of pioneer HuC/D-immunoreactive (ir) neurons that seemed to delaminate from the olfactory pit epithelium and migrate toward the telencephalon before the olfactory nerve was identifiable. A distinct, transient cell population, namely the migratory mass, courses later on in apposition to the developing olfactory nerve. It contains olfactory ensheathing glial (GFAP-ir) cells and HuC/D-ir neurons, some of which course toward an extrabulbar region. We also demonstrate that Pax6-ir cells coursing along the developing olfactory pathways in S. canicula are young migrating (HuC/D and DCX-ir) neurons of the migratory mass that do not form part of the terminal nerve pathway. Evidences that these Pax6 neurons originate in the olfactory epithelium are also reported. As Pax6 neurons in the olfactory epithelium show characteristics of olfactory receptor neurons, and migrating Pax6-ir neurons formed transient corridors along the course of olfactory axons at the entrance of the olfactory bulb, we propose that these neurons could play a role as guideposts for axons of olfactory receptor neurons growing toward the olfactory bulb.

Site-specific regulation of adult neurogenesis by dietary fatty acid content, vitamin E and flight exercise in European starlings

Exercise is known to have a strong effect on neuroproliferation in mammals ranging from rodents to humans. Recent studies have also shown that fatty acids and other dietary supplements can cause an upregulation of neurogenesis. It is not known, however, how exercise and diet interact in their effects on adult neurogenesis. We examined neuronal recruitment in multiple telencephalic sites in adult male European starlings (Sturnus vulgaris) exposed to a factorial combination of flight exercise, dietary fatty acids and antioxidants. Experimental birds were flown in a wind tunnel following a training regime that mimicked the bird's natural flight behaviour. In addition to flight exercise, we manipulated the composition of dietary fatty acids and the level of enrichment with vitamin E, an antioxidant reported to enhance neuronal recruitment. We found that all three factors - flight exercise, fatty acid composition and vitamin E enrichment - regulate neuronal recruitment in a site-specific manner. We also found a robust interaction between flight training and vitamin E enrichment at multiple sites of neuronal recruitment. Specifically, flight training was found to enhance neuronal recruitment across the telencephalon, but only in birds fed a diet with a low level of vitamin E. Conversely, dietary enrichment with vitamin E upregulated neuronal recruitment, but only in birds not flown in the wind tunnel. These findings indicate conserved modulation of adult neurogenesis by exercise and diet across vertebrate taxa and indicate possible therapeutic interventions in disorders characterized by reduced adult neurogenesis.

Anatomically discrete sex differences and enhancement by testosterone of cell proliferation in the telencephalic ventricle zone of the adult canary brain

Previous work in songbirds has suggested that testosterone increases neuronal recruitment and survival in HVC but does not affect neuronal proliferation in the ventricular zone and that males and females have similar rates of proliferation except at discrete locations. Many of these conclusions are however based on limited data or were inferred indirectly. Here we specifically tested the effects of testosterone on cellular proliferation in the ventricular zone of both male and female adult canaries. We implanted adult birds of both sexes with testosterone or empty implants for 1 week and injected them with BrdU. One day later, we collected their brains and quantified BrdU-positive cells in the ventricular zone (VZ) at different rostro-caudal levels of the brain, ranging from the level where the song nucleus Area X occurs through the caudal extent of HVC. Proliferation in the dorsal part of the VZ was low and unaffected by sex or testosterone treatment. In the ventral part of the VZ, females had more proliferating cells than males, but only at rostral levels, near Area X. Also in the ventral part of the VZ, testosterone increased proliferation in birds of both sexes, but only in the mid- to caudal-VZ, caudal to the level of Area X, around the septum and HVC. We thus demonstrate here that there is both an effect of testosterone and possibly a more subtle effect of sex on cellular proliferation in the adult songbird brain, and that these effects are specific to different levels of the brain.

Birth of neural progenitors during the embryonic period of sexual differentiation in the Japanese quail brain

Several brain areas in the diencephalon are involved in the activation and expression of sexual behavior, including in quail the medial preoptic nucleus (POM). However, the ontogeny of these diencephalic brain nuclei has not to this date been examined in detail. We investigated the ontogeny of POM and other steroid-sensitive brain regions by injecting quail eggs with 5-bromo-2-deoxyuridine (BrdU) at various stages between embryonic day (E)3 and E16 and killing animals at postnatal (PN) days 3 or 56. In the POM, large numbers of BrdU-positive cells were observed in subjects injected from E3-E10, the numbers of these cells was intermediate in birds injected on E12, and most cells were postmitotic in both sexes on E14-E16. Injections on E3-E4 labeled large numbers of Hu-positive cells in POM. In contrast, injections performed at a later stage labeled cells that do not express aromatase nor neuronal markers such as Hu or NeuN in the POM and other steroid-sensitive nuclei and thus do not have a neuronal phenotype in these locations, contrary to what is observed in the telencephalon and cerebellum. No evidence could also be collected to demonstrate that these cells have a glial nature. Converging data, including the facts that these cells divide in the brain mantle and express proliferating cell nuclear antigen (PCNA), a cell cycling marker, indicate that cells labeled by BrdU during the second half of embryonic life are slow-cycling progenitors born and residing in the brain mantle. Future research should now identify their functional significance.

Comparative aspects of adult neural stem cell activity in vertebrates

At birth or after hatching from the egg, vertebrate brains still contain neural stem cells which reside in specialized niches. In some cases, these stem cells are deployed for further postnatal development of parts of the brain until the final structure is reached. In other cases, postnatal neurogenesis continues as constitutive neurogenesis into adulthood leading to a net increase of the number of neurons with age. Yet, in other cases, stem cells fuel neuronal turnover. An example is protracted development of the cerebellar granular layer in mammals and birds, where neurogenesis continues for a few weeks postnatally until the granular layer has reached its definitive size and stem cells are used up. Cerebellar growth also provides an example of continued neurogenesis during adulthood in teleosts. Again, it is the granular layer that grows as neurogenesis continues and no definite adult cerebellar size is reached. Neuronal turnover is most clearly seen in the telencephalon of male canaries, where projection neurons are replaced in nucleus high vocal centre each year before the start of a new mating season--circuitry reconstruction to achieve changes of the song repertoire in these birds? In this review, we describe these and other examples of adult neurogenesis in different vertebrate taxa. We also compare the structure of the stem cell niches to find common themes in their organization despite different functions adult neurogenesis serves in different species. Finally, we report on regeneration of the zebrafish telencephalon after injury to highlight similarities and differences of constitutive neurogenesis and neuronal regeneration.

Influence of testosterone metabolites on song-control system neuroplasticity during photostimulation in adult European starlings (Sturnus vulgaris)

The song-control system is a network of discrete nuclei in the songbird brain that controls the production and learning of birdsong and exhibits some of the best-studied neuroplasticity found in the adult brain. Photoperiodic growth of the song-control system during the breeding season is driven, at least in part, by the gonadal steroid testosterone. When acting on neural tissue, however, testosterone can be metabolized into 5α-dihydrotestosterone (DHT) or 17β-estradiol (E2), which activate different hormonal signaling pathways. By treating adult starlings with both testosterone metabolites and metabolite antagonists, we attempted to isolate the effects of androgen and estrogen treatment on neuroplasticity during photostimulation in male and female European starlings (Sturnus vulgaris). Photostimulation resulted in a large HVC volume typical of the breeding season in all treatments independent of hormone treatment. E2 had additional effects on HVC growth by reducing neuron density and enhancing early survival of new neurons recruited to HVC in females but did not significantly affect HVC volume. Conversely, DHT reduced the migration of new neurons, assessed by the expression of doublecortin, to HVC. DHT also increased syrinx mass and maintained RA (robust nucleus of the arcopallium) cytoarchitecture in the presence of aromatase inhibitors. In addition, we document the first evidence of sex-specific neuroplastic responses of the song-control system to androgens and estrogens. These findings suggest that the contributions of DHT and E2 signaling in songbird neuroplasticity may be regulated by photoperiod and that future studies should account for species and sex differences in the brain.

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.

Postnatal changes in the distribution and density of neuronal nuclei and doublecortin antigens in domestic chicks (Gallus domesticus)

To understand better the rate of neurogenesis and the distribution of new neurons in posthatch domestic chicks, we describe and compare the expression of the neuronal nuclei protein (NeuN, a.k.a. Fox-3) and doublecortin antigens in the whole brain of chicks 2 days, 8 days, and 14 weeks posthatch. In the forebrain ventricular and paraventricular zones, the density of bromodeoxyuridine-, NeuN-, and doublecortin-labeled cells was compared between chicks 24 hours and 7 days after an injection of bromodeoxyuridine (2 and 8 days posthatch, respectively). The distribution of NeuN-labeled neurons was similar to Nissl-stained tissue, with the exception of some areas where neurons did not express NeuN: cerebellar Purkinje cells and olfactory bulb mitral cells. The ventral tegmental area of 2-day-old chicks was also faintly labeled. The distribution of doublecortin was similar at all timepoints, with doublecortin-labeled profiles located throughout all forebrain areas as well as in the cerebellar granule cell layer. However, doublecortin labeling was not detectable in any midbrain or brainstem areas. Our data indicate that a significant number of new neurons is still formed in the telencephalon of posthatch domestic chicks, whereas subtelencephalic areas (except for the cerebellum) finish their neuronal expansion before hatching. Most newly formed cells in chicks leave the paraventricular zone after hatching, but a pool of neurons stays in the vicinity of the ventricular zone and matures in situ within 7 days. Proliferating cells often migrate laterally along forebrain laminae into still-developing brain areas.

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.

Neurogenesis in the procerebrum of the snail Helix aspersa: a quantitative analysis

The procerebrum, a specialized structure for olfaction in terrestrial pulmonate molluscs, contains 20,000 to 50,000 small, uniformly sized neurons that increase in number with age. Here I show the likely source of neurons added to the procerebrum of Helix aspersa and that the rate of neuron addition depends on snail weight. After hatching, during the initial exponential growth phase, H. aspersa adds neurons to the procerebral apex by mitosis and from a cerebral tube. In the logistic growth phase beginning 30-40 days post-hatch, neurons also seem to be added to the procerebrum from the peritentacular and olfactory nerves, causing the rate of neuron addition to approximately double; but as in the earlier exponential growth phase, this rate remains a function of snail weight. This neuron addition throughout the life of the snail can be predicted by snail weight. In the two growth phases, the number of neurons in the procerebrum is given by logarithmic functions of snail weight. The results here for H. aspersa provide the basis for experiments to determine the peripheral origin and destination of neuronal precursors that are added to the procerebrum and to determine how neuron addition affects the function of the procerebrum.

A customizable 3-dimensional digital atlas of the canary brain in multiple modalities

Songbirds are well known for their ability to learn their vocalizations by imitating conspecific adults. This uncommon skill has led to many studies examining the behavioral and neurobiological processes involved in vocal learning. Canaries display a variable, seasonally dependent, vocal behavior throughout their lives. This trait makes this bird species particularly valuable to study the functional relationship between the continued plasticity in the singing behavior and alterations in the anatomy and physiology of the brain. In order to optimally interpret these types of studies, a detailed understanding of the brain anatomy is essential. Because traditional 2-dimensional brain atlases are limited in the information they can provide about the anatomy of the brain, here we present a 3-dimensional MRI-based atlas of the canary brain. Using multiple imaging protocols we were able to maximize the number of detectable brain regions, including most of the areas involved in song perception, learning, and production. The brain atlas can readily be used to determine the stereotactic location of delineated brain areas at any desirable head angle. Alternatively the brain data can be used to determine the ideal orientation of the brain for stereotactic injections, electrophysiological recordings, and brain sectioning. The 3-dimensional canary brain atlas presented here is freely available and is easily adaptable to support many types of neurobiological studies, including anatomical, electrophysiological, histological, explant, and tracer studies.