Change of spiny neuron structure in the basal ganglia song circuit and its regulation by miR-9 during song development.

Juvenile zebra finches learn to sing by imitating conspecific songs of adults during a sensitive period early in life. Area X is a basal ganglia nucleus of the song control circuit specialized for song-related sensory-motor learning during song development. The structural plasticity and the molecular mechanisms regulating neuronal structure in Area X during song development and maturation are unclear. In this study, we examined the structure of spiny neurons, the main neuron type in Area X, at key stages of song development in male zebra finches. We report that dendritic arbor of spiny neurons expands during the sensitive period for song learning, and this initial growth is followed by pruning of dendrites and spines accompanied by changes in spine morphology as the song circuit matures. Previously, we showed that overexpression of miR-9 in Area X impairs song learning and performance and alters the expression of many genes that have important roles in neuronal structure and function (Shi et al., 2018). As an extension of that study, we report here that overexpression of miR-9 in spiny neurons in juvenile zebra finches reduces dendritic arbor complexity and spine density in a developmental stage-specific manner. We also show that miR-9 regulates structural maintenance of spiny neurons in adulthood. Together, these findings reveal dynamic microstructural changes in the song circuit during the sensitive period of song development and provide evidence that miR-9 regulates neuronal structure during song development and maintenance.Significance Statement Song development in juvenile zebra finches provides a model to study sensitive period plasticity for language development and related neural developmental disorders in humans. Area X is a basal ganglia nucleus essential for song-related sensory-motor learning in the zebra finch. We show that dendritic arbor of spiny neurons in Area X undergoes an initial growth and expansion followed by pruning of dendrites and spines during song development, and that this process is regulated by miR-9 in a developmental stage specific manner. These findings reveal the temporal profiles of structural development of key neurons in the basal ganglia song circuit and reveal a possible molecular mechanism for restricting sensitive period plasticity during vocal development.