Developmental and Experience-dependent Synaptic Remodeling of a Song-control Brain Region in the Zebra Finch

Abstract

During a developmental critical period, juvenile male zebra finches learn to sing a highly stereotyped song from a tutor bird. Song learning in zebra finches shares many common behavioral, physiological, and neuroanatomical features with other nervous systems that exhibit critical periods. These include development of binocular vision in cat, formation of sensory maps in rodent, and acquisition of language in humans. One important regulatory principle summarized from the various systems is the experience-dependent structural remodeling within the associated brain areas. The brain area HVC, which is the vocal motor cortex analog in the zebra finch brain, plays a crucial role during song learning in juveniles and actively drives production of the learned song in adulthood. Similar to the structural remodeling that has been observed in primary visual cortex (V1) in cat and barrel cortex in rodents, dendritic spines in HVC undergo rapid remodeling in response to critical sensory experience. However, the mechanisms underlying the experience-dependent remodeling at the synaptic level are unclear. In particular, the HVC inhibitory circuit, which is shown to be crucial for song learning, has not been studied anatomically at the synaptic level yet. And the experience-dependent structural changes in HVC has not been carefully decoupled from developmental structural changes. Therefore, I set out to carefully study the structural synapse changes in HVC during song development.I propose two experimental approaches to investigate the open scientific questions described above. The first experiment was inspired by an in vivo imaging study, which reported initial sensory experience of the tutor song triggered spine stabilization and enhancement overnight in HVC in juvenile zebra finches (Roberts et al. 2010). I further investigated this experience-dependent structural remodeling in HVC in two ways. First, I further manipulated the song sensory experiences in juvenile zebra finches. In different groups of juveniles, I provided short, long, or no tutor exposure, and then harvested their HVCs for neurostructural analysis at the same age. I then examined HVC further with serial-section and block-face electron microscopy, which allowed me to directly and clearly identify, classify, and morphologically reconstruct HVC synapses. The results showed synaptic pruning, and suggested synaptic strengthening in HVC excitatory synapses in a song sensory experience-dependent manner, which was in agreement with previous findings. Surprisingly, the results also suggested a transient density increase of the inhibitory synapses in HVC, following one day of tutor song sensory exposure. Rapid experience-dependent remodeling of the inhibitory circuit in HVC has not been reported before, but similar has been observed in the mouse somatosensory cortex (Knott et al. 2002). Taken together, these results indicate that tutor song exposure during song learning has short- and long-term effects on both the excitatory and the inhibitory synaptic networks in HVC.In the second experiment, I investigated this experience-dependent structural remodeling at different developmental stages. I used two main groups of juvenile male zebra finches. In one group I deprived the juvenile completely from sensory exposure to the tutor song, while in the other group I provided the bird regular and controlled tutor song exposure. I harvested HVC from the birds at different development stages, and examined the synaptic structure in HVC under the electron microscope. I repeatedly observed a peak-decline developmental pattern, in which neurostructural parameters such as brain volume, synapse density, and total synapse number first increase and then decrease. This developmental pattern, which had been observed in different brain regions and species, reflects the overproduction of neural connections in the early stages of postnatal development and structural pruning in the later stages (Herrmann and Bischof 1986; Murphy and Magness 1984). Tutor song exposure lead to relative pruning of excitatory and inhibitory synapses in HVC, in comparison to birds that have not received tutor exposure.Combined with previous findings, I provide a detailed picture of the developmental structural synaptic remodeling in HVC during song learning. These findings provide clues into how HVC’s synaptic networks might store a memory of tutor song.