I focused on examining the structural remodeling in HVC associated with song sensory experience and age in the current PhD project. Specifically, I aim to study the structural synapse remodeling in HVC under different learning conditions (i.e.
different level of tutor song exposure) and track this structural remodeling during song development.
On the basis of the previous findings on structural remodeling during the critical period, especially those in the local networks of the songbird HVC, and the changes in spine dynamics during song learning in zebra finches, I conducted two experiments. The first experiment was designed to study the structural synapse remodeling that occurred in HVC in juvenile zebra finches in response to sensory exposure to the tutor song. And, the second experiment was designed to further study and decouple the developmental and song experience-dependent structural synapse remodeling in HVC of juvenile zebra finches learning song.
Considering the findings of the common phenomenon of structural remodeling in response to different types of learning during critical periods, I hypothesize tutor
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song experience would trigger local structural remodeling in HVC, such as the formation or removal of synapses, changes in the structural balance of excitatory and inhibitory synapses, and/or changes in synaptic morphology. Moreover, I expect that the excitatory and inhibitory synapses in HVC would change in number and proportion significantly during song behavior development, and would be influenced by song sensory experience of the bird.
1.6.1. Experiment I
The primary goal of this experiment is to quantify the synapse density and the percentage of synapse subtypes (symmetric synapses for example) in HVC of juveniles subjected to different tutoring paradigms. I control the experimental variables so that different groups in Experiment I differ only in their sensory experience of the tutor song exposure. Therefore, I chose to harvest HVC samples from birds of all groups at the same development stage during the song learning period of zebra finches. As described in previous sections, the synapses in the corresponding brain region could react quickly and long-lastingly to critical sensory experiences. Thus, it was interesting for me to learn about how the synapses in HVC react to the tutor song exposure, in short term, and in long term. Whether the synapses in HVC of song-tutored birds differ from those of song-isolated birds, at the same developmental stage? Given the potential different roles during song learning, I wanted to learn about how the excitatory and inhibitory synapses in HVC react to such critical experiences. I proposed the following specific questions to be addressed by Experiment I:
1. Does the synapse density in HVC change as a result of sensory exposure to tutor song? And if so, how does the synapse density change in the short term (acute effect), and in the long term (chronic effect)? Do the excitatory and inhibitory synapses in HVC follow the same structural dynamics upon tutor song exposure?
2. Does the percentage of inhibitory synapses in HVC change as a result of sensory exposure to tutor song? And if so, how does the percentage of the inhibitory synapses react in short-term and long-term to the sensory exposure to the tutor song?
3. Does the size of the synapses in HVC change as a result of sensory exposure to tutor song? And how does the synapse change in size upon short-term and long-term sensory exposure to the tutor song?
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1.6.2. Experiment II
Experiment II was proposed as a follow-up experiment to Experiment I. HVC sample were collected from birds of all groups in Experiment I at the same age to eliminate developmental differences in the result. In experiment II, I further investigated both the developmental and experience-dependent structural remodeling in HVC of juvenile zebra finches. The primary goal of this experiment was to quantify the structural synaptic differences in HVC of juvenile birds subjected to different development stages (i.e. 30 dph, 60 dph, and 90 dph), with different tutor song experience (i.e. song-tutored or song-isolated). In the findings summarized in section 1.4, there were quite a number of experience-dependent structural synapse changes inside the adult brain, regulated similarly as in the juvenile brain. This made it more important to decouple the developmental structural changes from the experience-dependent structural changes of the brain, since the latter one seems persist into adulthood and could be governed by more general regulatory principles. Therefore, I wanted to take advantage of the well-defined and highly stereotyped song learning behavior in juvenile zebra finch as a model, to separate and investigate both the developmental and experience-dependent structural synapse remodeling. Specifically, with Experiment II I wanted to address the following questions:
1. Does the volume of HVC, synapse density, and synapse number in HVC change as a result of development while deprived from the critical sensory experience (song-isolated birds), and does it change as a result of normal song development (song-tutored birds)? What would be the difference resulted from song experience at given developmental stage (60 or 90 dph)?
2. Do the excitatory and inhibitory synapses grow simultaneously or follow different developmental patterns? Does the percentage of inhibitory synapses in HVC change during song development, and how would the song experience impact such changes?