Type 2 deiodinase in the developing song system of the zebra finch: a path to neuroplasticity?

The zebra finch (Taeniopygia guttata) song system consists of several interconnected brain nuclei that show a marked difference in size and development between males and females. While the young zebra finch brain already reaches adult size by 20-30 days, the song nuclei in young males continue to show a high degree of neuroplasticity during the song learning period until they reach adulthood and their song crystallises. Although a lot of research has been conducted on how these changes occur, the precise molecular mechanisms remain poorly understood. Thyroid hormones (THs) have been associated with neural development in several animals, but surprisingly have been mostly overlooked in songbirds. We hypothesised that THs play a role in the development of the song nuclei and consequently in the song learning process. We therefore performed in situ hybridisation for type 2 deiodinase (DIO2), the most important TH-activating enzyme in the brain, on both male and female zebra finch brains at multiple time points during the song learning process: 10, 20, 30, 40, 50, 60, 90 and 120 days post hatch (dph). These time points span the duration of the sensory phase (20-60 dph), the sensorimotor phase (30-90 dph) and the crystallisation phase (90-120 dph) of song learning. As female zebra finches do not learn to sing, they provide a good reference for the same brain regions with presumably minimal plastic changes. Our results show that DIO2 is expressed in the endothelial cells of the blood-brain barrier throughout the entire brain in both sexes at early stages (10-20 dph), correlating with the intense neuronal proliferation and differentiation at those stages. However, while the expression levels gradually become undetectable after 30 dph in almost the entire brain, DIO2 expression remains strong up to 60 dph in the endothelial cells in the nuclei Area X of the striatum, HVC (used as a proper name) and RA (robust nucleus of the arcopallium) of the male brain. The 30 dph time point marks the onset of the sensorimotor phase, in which the birds start producing their own song and several song nuclei start to increase in volume. By the time the song crystallises, DIO2 expression recedes entirely. In females however, DIO2 expression is limited to non-existent in the entire brain from 30 dph onwards. These results indicate there is a high degree of local thyroid hormone activation during the period for song learning in males and suggest that THs have a direct or indirect role in the development of the song nuclei. We can conclude that during the majority of both the sensory phase and the sensorimotor phase, THs are activated specifically in the endothelial cells of several song nuclei in males. This could either influence the development of the blood vessels, increasing neurotrophin expression and the influx of nutrients in the region, or the active TH could be transported to the surrounding neural tissue and induce plasticity through activation of TH-responsive genes.status: publishe