Adult neurogenesis is the process in which new neurons are born and subsequently integrate into functional brain circuits after birth and into adulthood. Avian species including songbirds are among vertebrate species that demonstrate particularly robust adult neurogenesis throughout their telencephalon, in contrast with the more limited neurogenic potential that are observed in adult mammals after birth. Adult neurogenesis in songbirds is observed in brain circuits that underlie complex specialized behavior, including the song control system and the hippocampus. The degree of postnatal and adult neurogenesis in songbirds varies between species, shows sexual dimorphism, fluctuates seasonally, and depends on hormone levels, cell death rates, and social environment. The increased extent of adult neurogenesis in birds compared to other vertebrates, especially in circuits that underlie complex specialized behavior, makes birds an excellent animal model to study this process and its functionality. Methods used in research to track adult neurogenesis in birds include the use of thymidine analogues and identifying endogenous markers of neurogenesis. Historically, the discovery of adult neurogenesis in songbirds substantially contributed to establishing the presence of adult neurogenesis and to progressing a line of research tightly associated with many potential clinical applications.
Mechanism
General mechanism New neurons are born on the walls of the lateral ventricles (ventricular zone) and migrate to their destination where they integrate into the existing circuits. Neurogenesis occurs throughout the ventricular zone, but proliferation is especially high in specific locations in the lateral wall of the ventricle. Radial glia, a subset of vimentin-expressing glial cells that are the adult neural stem cells, reside in the dorsal and ventral ventricular zone and extend long processes that permeate the neural tissue. Radial glia asymmetrically divide to give rise to new neurons. Neurons born in the ventricular zone first migrate tangentially along the ventricular zone, then radially on radial glia processes to their destination. However, there is evidence that another form of migration is also in place. Indeed, some newborn neurons have been shown to undergo non-radial glia accommodated migration, where they do not move on the radial glia processes but rather "wander" around in a diffusion-like manner until they reach their destination.
Adult neurogenesis in different systems Adult neurogenesis in the avian brain occurs in many different pallial and subpallial regions, including the song nuclei, the hippocampus, and the olfactory regions. Even though adult neurogenesis is widespread across the telencephalon, all the regions where it is especially high are associated with the learning of new information, suggesting a possible function of adult neurogenesis to be offering an additional avenue of plasticity required during learning.
Neurogenesis in the song system Although the recruitment of new neurons is widespread in the avian telencephalon, songbirds exhibit particularly robust postnatal and adult neurogenesis in their song control system nuclei. The song control system is found exclusively in oscine birds and is responsible for the learning and production of songs. Neurogenesis observed in the song system brain areas is shown to be associated with the learning of new songs. Neurogenesis in songbirds was first identified and has been most well studied in the song control system.
Song system circuit
The song control system of songbirds consists of a collection of specialized brain areas (nuclei) that are involved in the learning and production of song. Those include the HVC (former "Higher Vocal Center", now HVC used as the proper name), a nucleus located in the posterior nidopallium, the nucleus robustus arcopallialis (RA), also located in the nidopallium, and Area X, a structure comparable to the mammalian striatum.
The song control system is divided into two main pathways: the motor pathway (or caudal pathway) and the anterior forebrain circuit. The motor pathway is responsible for song production. It includes projections from the HVC to the RA, and subsequently to parts of the medulla that control the syrinx (the songbird vocal organ) and respiration via the hypoglossal nucleus (nXIIts). Lesions in this pathway in adult birds lead to impaired song production. The anterior forebrain pathway is responsible for song learning and plasticity. It consists of projections from the HVC to Area X, then DLM, LMAN, and concludes in RA. In addition, LMAN also projects independently to RA, and this input is thought to be responsible for song variability. Lesions in this pathway in juvenile birds leads to impaired song learning.
Mechanism and migration Adult neurogenesis in the song control system nuclei is shown to be restricted to neurons in Area X, and the neurons of HVC projecting to RA. On the contrary, the neurons of HVC projecting to Area X do not regenerate and are born before hatching. There is also some evidence to suggest a regenerating interneuron population in the HVC. Most of the newborn neurons that migrate to the HVC fail to differentiate and integrate, but the ones that do survive in the circuit for a long time.
Neurogenesis in the hippocampus Adult neurogenesis also occurs in the hippocampal formation of songbirds. Postnatal neurogenesis in the avian hippocampus is especially prominent in species of songbirds that engage in seasonal food-caching behavior, such as the titmice and chickadees. Indeed, hippocampal neurogenesis among these species is strongly associated with memory demands related to food-storing and -retrieval. Hippocampal adult neurogenesis in chickadees seems to occur through neuronal turnover rather than a general increase in the number of neurons. This process of neural replacement is highly influenced by seasons and coincides with the increasing demand for food-caching behavior. For instance, new neurons that are specifically born in early autumn are more likely to integrate into the existing circuit and survive longer than new neurons born other times of the year. It has been suggested that the seasonal modulation of hippocampal neurogenesis among food-caching songbirds is mediated by photodriven endogenous timers. Conversely, it has also been suggested that the novel experiences brought upon by the change in season is the primary driver of the hippocampal neurogenic processes in songbirds.
Avian hippocampus The avian hippocampus is a forebrain structure subdivided into the V-shaped, the dorsomedial (DM), the dorsolateral (DL), and the triangular region (Tr). Even though there are substantial structural differences between the mammalian hippocampus and the avian hippocampus, there is evidence for subregion homology based on connectivity, gene expression patterns and development. A dominant homology model describes the V-shaped region of the avian hippocampus as homologous to the mammalian dentate gyrus (DG) and the DM region homologous to the cornus ammonis (CA).
Compensatory neurogenesis Compensatory neurogenesis occurs in response to injury of neural tissue. In the songbird, ablation of brain regions that physiologically undergo neurogenesis is shown to trigger enhanced restorative waves of neurogenesis, eventually restoring the singing behavior itself. The restorative mechanism is limited to the types of neurons that normally regenerate, namely the HVC neurons that projects to RA and the HVC interneurons. In contrast, the HVC neurons that project to area X do not undergo replacement in instances of ablation.
Factors underlying mechanism of adult neurogenesis in the songbird song system There are many factors shown to underlie adult neurogenesis in the song control systems in juvenile and adult songbirds (described in the sections below). Overall, the amount of postnatal and adult neurogenesis in songbirds varies between species, shows sexual dimorphism, fluctuates seasonally, and depends on hormone levels, cell death rates, and social environment.
Hormones and neurotrophic factors
Singing behavior and the song control system structures show high sexual dimorphism. Male songbirds sing more and have more song syllable variation than their female conspecifics. In parallel, their HVC and RA nuclei are considerably larger than in females. This dimorphism is shown to be under control of sex steroids: the song nuclei in male songbirds have high levels of testosterone and its metabolites, as well as restricted expression of the estrogen receptor ERα. Additionally, testosterone administration to female canaries leads to increased singing and enlarged HVC and RA. Testosterone however does not seem to enhance rate of proliferation itself, but rather the survival and integration success of already born neuroblasts. Furthermore, seasonal waves of neuron proliferation precedes peaks in testosterone, so neurons born during these waves benefit from the testosterone survival effects.
The effect of testosterone on new neuron survival however is not direct, but mediated by neurotrophic factors. Testosterone treatment increases levels of brain-derived neurotrophic factor (BDNF) in HVC, which in turn increases survival of newly generated HVC neurons. However, this effect is observed only when BDNF is administered between 14 - 20 days after neuronal birth. The proposed pathway is that testosterone is aromatized by brain aromatase to estradiol, which in turn increases expression of trophic factor VEGF and its receptor Quek1/VEGFRR2 in the endothelial cells. This results in local angiogenesis, as the number of endothelial cells and capillary diameter increases. VEGF binding on the increased endothelial VEGFRR2 receptors stimulates BNDF upregulation, which in turn increases new neuron survival.
Genes Genes related to angiogenesis or neurogenesis and migration related cytoskeletal rearrangements (e.g. neurite growth) are upregulated in neurogenic areas in the songbird brain. Many neurotrophins, including BDNF, IGF-1, and VEGF are highly expressed in seasons when adult neurogenesis in the HVC peaks.
FoxP2 (Forkhead box protein 2) is a transcription factor that in the songbird is associated with song learning. In the adult songbird brain, the adult-born neurons of Area X are intensely expressing FoxP2. Expression of FoxP2 in Area X neurons has an inverse relationship with testosterone levels. In the context of neurogenesis, FoxP2 is shown to play a role in cytoskeletal rearrangement, spine and dendrite formation. Knockdown of FoxP2 expression in young songbirds results in impaired song learning and reduced spine density, especially when the intervention occurs at the neural stem cell stage.
Other proteins that are upregulated in cells undergoing neurogenesis include proteins that are associated with DNA replication and mitosis. PCNA (proliferating cell nuclear antigen) is a protein that is expressed during DNA replication, and it is associated with DNA polymerase A activity.
Species Songbird species vary in their way of learning and modifying their songs. There are two types of learners: sensitive period learners (or "closed" learners) and open-ended learners (or "open" learners). The former, like chaffinches or zebra finches, learn their songs from a tutor during a time period in early life and show little modifications to it later in life, whilst the latter, like canaries, change their song each breeding season. Although there is adult neurogenesis throughout the lifespan of both types of learners, the amount of HVC neuron turnover is substantially higher in the "open" learners, reflecting the higher plasticity of their song.
Seasonality