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

Unilateral vocal nerve resection alters neurogenesis in the avian song system in a region-specific manner

New neurons born in the adult brain undergo a critical period soon after migration to their site of incorporation. During this time, the behavior of the animal may influence the survival or culling of these cells. In the songbird song system, earlier work suggested that adult-born neurons may be retained in the song motor pathway nucleus HVC with respect to motor progression toward a target song during juvenile song learning, seasonal song restructuring, and experimentally manipulated song variability. However, it is not known whether the quality of song per se, without progressive improvement, may also influence new neuron survival. To test this idea, we experimentally altered song acoustic structure by unilateral denervation of the syrinx, causing a poor quality song. We found no effect of aberrant song on numbers of new neurons in HVC, suggesting that song quality does not influence new neuron culling in this region. However, aberrant song resulted in the loss of left-side dominance in new neurons in the auditory region caudomedial nidopallium (NCM), and a bilateral decrease in new neurons in the basal ganglia nucleus Area X. Thus new neuron culling may be influenced by behavioral feedback in accordance with the function of new neurons within that region. We propose that studying the effects of singing behaviors on new neurons across multiple brain regions that differentially subserve singing may give rise to general rules underlying the regulation of new neuron survival across taxa and brain regions more broadly.

Seasonal changes in neuronal turnover in a forebrain nucleus in adult songbirds

Neuronal death and replacement, or neuronal turnover, in the adult brain are one of many fundamental processes of neural plasticity. The adult avian song control circuit provides an excellent model for exploring mature neuronal death and replacement by new neurons. In the song control nucleus, HVC of adult male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelli) nearly 68,000 neurons are added each breeding season and die during the subsequent nonbreeding season. To accommodate large seasonal differences in HVC neuron number, the balance between neuronal addition and death in HVC must differ between seasons. To determine whether maintenance of new HVC neurons changes within and between breeding and nonbreeding conditions, we pulse-labeled two different cohorts of new HVC neurons under both conditions and quantified their maintenance. We show that the maintenance of new HVC neurons, as well as new nonneuronal cells, was higher at the onset of breeding conditions than at the onset of nonbreeding conditions. Once a steady-state HVC volume and neuronal number were attained in either breeding or nonbreeding conditions, neuronal and nonneuronal maintenance were similarly low. We found that new neuronal number correlated with a new nonneuronal number within each cohort of new neurons. Together, these data suggest that sex steroids promote the survival of an initial population of new neurons and nonneuronal cells entering HVC. However, once HVC is fully grown or regressed, neuronal and nonneuronal cell turnover is regulated by a common mechanism likely independent of direct sex steroid signaling.

Adult Neurogenesis Leads to the Functional Reconstruction of a Telencephalic Neural Circuit

Seasonally breeding songbirds exhibit pronounced annual changes in song behavior, and in the morphology and physiology of the telencephalic neural circuit underlying production of learned song. Each breeding season, new adult-born neurons are added to the pallial nucleus HVC in response to seasonal changes in steroid hormone levels, and send long axonal projections to their target nucleus, the robust nucleus of the arcopallium (RA). We investigated the role that adult neurogenesis plays in the seasonal reconstruction of this circuit. We labeled newborn HVC neurons with BrdU, and RA-projecting HVC neurons (HVCRA) with retrograde tracer injected in RA of adult male white-crowned sparrows (Zonotrichia leucophrys gambelii) in breeding or nonbreeding conditions. We found that there were many more HVCRA neurons in breeding than nonbreeding birds. Furthermore, we observed that more newborn HVC neurons were back-filled by the tracer in breeding animals. Behaviorally, song structure degraded as the HVC-RA circuit degenerated, and recovered as the circuit regenerated, in close correlation with the number of new HVCRA neurons. These results support the hypothesis that the HVC-RA circuit degenerates in nonbreeding birds, and that newborn neurons reconstruct the circuit in breeding birds, leading to functional recovery of song behavior.

SIGNIFICANCE STATEMENT

We investigated the role that adult neurogenesis plays in the seasonal reconstruction of a telencephalic neural circuit that controls song behavior in white-crowned sparrows. We showed that nonbreeding birds had a 36%-49% reduction in the number of projection neurons compared with breeding birds, and the regeneration of the circuit in the breeding season is due to the integration of adult-born projection neurons. Additionally, song structure degraded as the circuit degenerated and recovered as the circuit regenerated, in close correlation with new projection neuron number. This study demonstrates that steroid hormones can help reestablish functional neuronal circuits following degeneration in the adult brain and shows non-injury-induced degeneration and reconstruction of a neural circuit critical for producing a learned behavior.

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.

GSK-3β Inhibition Affects Singing Behavior and Neurogenesis in Adult Songbirds

GSK-3 (glycogen synthase kinase-3) is a serine/threonine kinase which is a critical regulator in neuronal signaling, cognition, and behavior. We have previously shown that unlike other vertebrates that harbor both α and β GSK-3 genes, the α gene is missing in birds. Therefore, birds can be used as a new animal model to study the roles of GSK-3β in behavior and in regulating adult neurogenesis. In the present study, we inhibited GSK-3β in brains of adult male zebra finches (Taeniopygia guttata) and accordingly investigated how this inhibition affects behavior and cell proliferation. Our results show that GSK-3 inhibition: (1) affects specific aspects of singing behavior, which might be related to social interactions in birds, and (2) differentially affects cell proliferation in various parts of the ventricular zone. Taken together, our study demonstrates a role of GSK-3β in regulating singing behavior and neuronal proliferation in birds and highlights the importance of GSK-3β in modulating cognitive abilities as well as social behavior.

Transsynaptic trophic effects of steroid hormones in an avian model of adult brain plasticity

The avian song control system provides an excellent model for studying transsynaptic trophic effects of steroid sex hormones. Seasonal changes in systemic testosterone (T) and its metabolites regulate plasticity of this system. Steroids interact with the neurotrophin brain-derived neurotrophic factor (BDNF) to influence cellular processes of plasticity in nucleus HVC of adult birds, including the addition of newborn neurons. This interaction may also occur transsynpatically; T increases the synthesis of BDNF in HVC, and BDNF protein is then released by HVC neurons on to postsynaptic cells in nucleus RA where it has trophic effects on activity and morphology. Androgen action on RA neurons increases their activity and this has a retrograde trophic effect on the addition of new neurons to HVC. The functional linkage of sex steroids to BDNF may be of adaptive value in regulating the trophic effects of the neurotrophin and coordinating circuit function in reproductively relevant contexts.

Reactive neurogenesis in response to naturally occurring apoptosis in an adult brain

Neuronal birth and death are tightly coordinated to establish and maintain properly functioning neural circuits. Disruption of the equilibrium between neuronal birth and death following brain injury or pharmacological insult often induces reactive, and in some cases regenerative, neurogenesis. Many neurodegenerative disorders are not injury-induced, however, so it is critical to determine if and how reactive neurogenesis occurs under noninjury-induced neurodegenerative conditions. Here, we used a model of naturally occurring neural degradation in a neural circuit that controls song behavior in Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) and examined the temporal dynamics between neuronal birth and death. We found that during seasonal-like regression of the song, control nucleus HVC (proper name), caspase-mediated apoptosis increased within 2 d following transition from breeding to nonbreeding conditions and neural stem-cell proliferation in the nearby ventricular zone (VZ) increased shortly thereafter. We show that inhibiting caspase-mediated apoptosis in HVC decreased neural stem-cell proliferation in the VZ. In baseline conditions the extent of neural stem-cell proliferation correlated positively with the number of dying cells in HVC. We demonstrate that as apoptosis increased and the number of both recently born and pre-existing neurons in HVC decreased, the structure of song, a learned sensorimotor behavior, degraded. Our data illustrate that reactive neurogenesis is not limited to injury-induced neuronal death, but also can result from normally occurring degradation of a telencephalic neural circuit.

The singer and the song: the neuromechanics of avian sound production

Song is crucial to songbirds for establishing territories and signaling genetic quality and an important driver in speciation. Songbirds also have become a widely used experimental model system to study the neural basis of vocal learning, a form of imitation learning with strong parallels to human speech learning. While there is a strong focus on central processing of song production, we still have limited insights into the functional output of the motor neural circuits. This review focuses on recent developments in motor control, biomechanics and feedback mechanisms of sound production in songbirds.

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.

Wild-type but not mutant SOD1 transgenic astrocytes promote the efficient generation of motor neuron progenitors from mouse embryonic stem cells

Background

The efficient derivation of mature (Hb9+) motor neurons from embryonic stem cells is a sought-after goal in the understanding, and potential treatment, of motor neuron diseases. Conditions that promote the robust generation of motor neuron progenitors from embryonic stem cells and that promote the survival of differentiated motor neurons ex vivo are likely, therefore, to be critical in future biological/therapeutic/screening approaches. Previous studies have shown that astrocytes have a protective effect on differentiated motor neurons (in vivo and ex vivo), but it remains unclear whether astrocytes also play a beneficial role in the support of motor neuron progenitors. Here we explore the effect of murine astrocyte-conditioned medium on monolayer cultures of mouse embryonic stem cell-derived motor neuron progenitors.

Results

Our data show that wild-type astrocyte-conditioned medium significantly increases the number of Olig2+/Hb9- progenitors, which subsequently differentiate into Hb9+/Islet1+ post-mitotic motor neurons. Intriguingly, while astrocyte-conditioned medium derived from mice transgenic for wild-type human SOD1 mimics the effect of wild-type astrocytes, conditioned medium derived from astrocytes carrying an amyotrophic lateral sclerosis-related SOD1-G93A mutation shows no such beneficial effect. The effect of astrocyte-conditioned medium, moreover, is specific to motor neurons: we find that interneurons generated from mouse embryonic stem cells are unaffected by conditioned media from any type of astrocyte.

Conclusions

Our study indicates that conditioned medium derived from wild type astrocytes enhances the efficient generation of motor neurons from mouse embryonic stem cells by enhancing motor neuron progenitors. In contrast, conditioned medium from SOD1-G93A astrocytes does not show a similar enhancing effect.

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.