Comparative study on song behavior, and ultra-structural, electrophysiological and immunoreactive properties in RA among deafened, untutored and normal-hearing Bengalese finches

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.

Neural circuit-wide analysis of changes to gene expression during deafening-induced birdsong destabilization

Sensory feedback is required for the stable execution of learned motor skills, and its loss can severely disrupt motor performance. The neural mechanisms that mediate sensorimotor stability have been extensively studied at systems and physiological levels, yet relatively little is known about how disruptions to sensory input alter the molecular properties of associated motor systems. Songbird courtship song, a model for skilled behavior, is a learned and highly structured vocalization that is destabilized following deafening. Here, we sought to determine how the loss of auditory feedback modifies gene expression and its coordination across the birdsong sensorimotor circuit. To facilitate this system-wide analysis of transcriptional responses, we developed a gene expression profiling approach that enables the construction of hundreds of spatially-defined RNA-sequencing libraries. Using this method, we found that deafening preferentially alters gene expression across birdsong neural circuitry relative to surrounding areas, particularly in premotor and striatal regions. Genes with altered expression are associated with synaptic transmission, neuronal spines, and neuromodulation and show a bias toward expression in glutamatergic neurons and Pvalb/Sst-class GABAergic interneurons. We also found that connected song regions exhibit correlations in gene expression that were reduced in deafened birds relative to hearing birds, suggesting that song destabilization alters the inter-region coordination of transcriptional states. Finally, lesioning LMAN, a forebrain afferent of RA required for deafening-induced song plasticity, had the largest effect on groups of genes that were also most affected by deafening. Combined, this integrated transcriptomics analysis demonstrates that the loss of peripheral sensory input drives a distributed gene expression response throughout associated sensorimotor neural circuitry and identifies specific candidate molecular and cellular mechanisms that support the stability and plasticity of learned motor skills.

DARPP-32 distinguishes a subset of adult-born neurons in zebra finch HVC

Adult male zebra finches (Taeniopygia guttata) continually incorporate adult-born neurons into HVC, a telencephalic brain region necessary for the production of learned song. These neurons express activity-dependent immediate early genes (e.g., zenk and c-fos) following song production, suggesting that these neurons are active during song production. Half of these adult-born HVC neurons (HVC NNs) can be backfilled from the robust nucleus of the arcopallium (RA) and are a part of the vocal motor pathway underlying learned song production, but the other half do not backfill from RA, and they remain to be characterized. Here, we used cell birth-dating, retrograde tract tracing, and immunofluorescence to demonstrate that half of all HVC NNs express the phosphoprotein DARPP-32, a protein associated with dopamine receptor expression. We also demonstrate that DARPP-32+ HVC NNs are contacted by tyrosine hydroxylase immunoreactive fibers, suggesting that they receive catecholaminergic input, have transiently larger nuclei than DARPP-32-neg HVC NNs, and do not backfill from RA. Taken together, these findings help characterize a group of HVC NNs that have no apparent projections to RA and so far have eluded positive identification other than HVC NN status.

A role for nonapeptides and dopamine in nest-building behaviour

During nest building in zebra finches (Taeniopygia guttata), several regions in the social behaviour network and the dopaminergic reward system, which are two neural circuits involved in social behaviour, appear to be active in male and female nest-building finches. Because the nonapeptides, mesotocin and vasotocin and the neurotransmitter, dopamine, play important roles in avian social behaviour, we tested the hypothesis that mesotocinergic-vasotocinergic and dopaminergic neuronal populations in the social behaviour network and dopaminergic reward system, respectively, are active during nest building. We combined immunohistochemistry for Fos (an indirect marker of neuronal activity) and vasotocin, mesotocin or tyrosine hydroxylase on brain tissue from nest-building and non-nest-building male and female zebra finches and compared Fos immunoreactivity in these neuronal populations with the variation in nest-building behaviour. Fos immunoreactivity in all three types of neuronal populations increased with some aspect of nest building: (i) higher immunoreactivity in a mesotocinergic neuronal population of nest-building finches compared to controls; (ii) increased immunoreactivity in the vasotocinergic neuronal populations in relation to the amount of material picked up by nest-building males and the length of time that a male spent in the nest with his mate; and (iii) increased immunoreactivity in a dopaminergic neuronal population in relation to the length of time that a male nest-building finch spent in the nest with his mate. Taken together, these findings provide evidence for a role of the mesotocinergic-vasotocinergic and dopaminergic systems in avian nest building.

Ultrastructural and electrophysiological analysis of Area X in the untutored and deafened Bengalese finch in relation to normally reared birds

Birdsong learning bears many similarities to human speech acquisition. Although the anterior forebrain pathway (AFP) is believed to be involved in birdsong learning, the underlying neural mechanisms are unclear. We produced two types of abnormal song learning: young birds untutored from adult "song tutors", or birds deafened by bilateral cochlear removal before the onset of sensory learning. We then studied how ultrastructure and electrophysiological activity changed in an AFP nucleus, Area X, among these birds at adulthood. Our results showed that, although the size of Area X did not change significantly, the numbers of synapses per unit area and compound synapses and the percent of concave synapses increased significantly in the untutored or deafened birds. The percent of perforated synapses or axo-spinous synapses decreased compared to the normally reared birds, suggesting a decreased efficiency of synaptic transmission in the untutored or deafened birds. We then identified several types of spontaneously firing cells in Area X. Cells with fast and slow firing rates did not show significant electrophysiological differences among the groups, but cells with moderate firing rates, most likely DLM-projecting neurons, fired at significantly lower rates in the untutored and deafened birds. In addition, cells firing irregularly were only found in the deafened birds. Thus, the decreased or irregular electrophysiological activity in the untutored or deafened birds, together with the corresponding ultrastructural findings, could be implicated in the abnormal song production in these two types of birds.

Changes in ultra-structures and electrophysiological properties in HVC of untutored and deafened Bengalese finches relation to normally reared birds: implications for song learning

Songbirds are increasingly used as an experimentally tractable system to study the neurobiological underpinnings of vocal learning. To gain additional insights into how birdsongs are learned, we compared the size of HVC, the high vocal center for song production, and its ultrastructural or electrophysiological properties between the normally reared Bengalese finches, and the untutored or deafened ones before the onset of sensory learning (around post-hatching day 20). Our results showed that HVC had more synapses and concave synaptic curvature, but fewer perforated synapse, in the untutored or deafened birds in comparison with those in the normally reared birds. Although there was no significant difference of the ratio of straight or compound synapses, there was an increasing tendency for the untutored and deafened birds to possess more straight and compound synapses. These data revealed that synapses in the isolated or deafened birds had lower synapse activity in relation to those with normal hearing. This was confirmed by our electrophysiological results to show significant decreases in the firing rates of spike or burst in the isolated or deafened birds in the three types of HVC neurons i.e., putative X-projecting neurons, RA-projecting neurons and interneurons. In addition, low firing frequency (<10Hz) occurred much more in the above three types of HVC neurons in the tutored or deafened birds than in the normally reared birds. These data suggest that all the three putative types of neurons in HVC might be involved in the activity of the production of adult normal songs.