Author:

Keywords:

Neuroplasticity, Thyroid hormones, Brain development, Song control system

Abstract:

During juvenile development, male songbirds exhibit a high amount of neuroplasticity in the brain regions responsible for song learning and production, called the 'song control nuclei'. Additionally, seasonal learners display a cycle of song control nucleus growth and regression throughout a year: song control nuclei increase in volume when the days get longer in early spring and regress again when days get shorter. Both neuroplastic phenomena are associated with song learning. In zebra finches for example, developmental neuroplasticity is accompanied by the learning of a single song that does not change after they become adults. In contrast, the brain of strong, seasonal learners like European starlings becomes sensitive again to new input every spring, which is associated with song control nucleus growth. Song learning and nucleus plasticity happen more strongly or even exclusively in male birds. Both displays of neuroplasticity (developmental and seasonal) are rare in the animal kingdom and their correlation with speech learning is obvious, making it a highly interesting research topic. Despite the appeal of the phenomena as a basis for research for human applications, the molecular and cellular mechanisms underlying this type of neuroplasticity are still unclear. In this thesis we investigated the possible involvement of thyroid hormones (THs) as regulators in this learning-associated plastic process. THs are essential for many processes and not in the least for neural development, differentiation and maturation. The effects of THs in a given tissue are mainly regulated by the local availability of three types of regulators. TH transporters enable THs to cross the blood-brain barrier and to enter neurons and glial cells through their cell membrane. The four known avian TH transporters are the monocarboxylate transporters 8 and 10, the organic anion-transporting polypeptide 1C1 and the L-type amino acid transporter 1 (LAT1). Deiodinases maintain the necessary amount of active TH by converting the prohormone T4 to the bioactive hormone T3 (deiodinase type 2, DIO2), or by inactivating T4 and T3 (deiodinase type 3, DIO3). Finally, TH receptors (encoded by the THRA and THRB genes) are ligand dependent transcription factors that regulate expression of TH-responsive genes. We used in situ hybridization to study the presence of mRNA of these regulators; firstly in the developing zebra finch brain during the entire period of song learning (10 to 120 days post hatch (dph)) and secondly in the brain of seasonally learning starlings in various photoperiods: photosensitive, photostimulated and photorefractory, corresponding to winter, early spring and summer respectively. In the first part of our research, we showed that in zebra finches, DIO2 expression was high in the endothelial cells lining the brain capillaries in the entire telencephalon at 10 and 20 dph. At 30 dph, when song control nuclei are still growing but the rest of the brain has attained its mature size, DIO2 expression diminished everywhere except in HVC, RA and Area X of male birds. Said expression remained high up to 60 dph while expression in the rest of the telencephalon became undetectable. At 90 and 120 dph, when song crystallizes, DIO2 expression in the nuclei also became nearly undetectable. In females, no such local expression was observed; DIO2 expression inside the song control nuclei receded in parallel with that in the rest of the brain at 20-30 dph. Additionally, in males but not females, LAT1 expression was higher in HVC than in the rest of the brain from 30 dph up to our final sampling point at 120 dph. THRA expression was widespread and THRB expression was generally low except in RA in both sexes. Our data clearly indicate that TH regulation, of which mainly TH activation by DIO2, is gender-specific and dynamic over time during developmental neuroplasticity. Secondly, we showed that in seasonal neuroplasticity, DIO3 expression in the starling HVC was strongly elevated after 4 weeks of photostimulation compared to the photosensitive state. This was accompanied by a decrease in expression of LAT1, further substantiating that TH action is actively restricted at that point in the photostimulated state. Assuming HVC has reached its maximal size by that time, we suggest that DIO3 inhibits further TH action and thus neuroplasticity in HVC. This would help in keeping the nucleus stable during breeding, which is necessary for strong, stereotypic song. DIO2 expression was below the detection limit at all stages but may yet be expressed in HVC shortly after photostimulation and thus before our studied time point. Both THRA and THRB were expressed but the observed expression level was constant for the different photoperiods. The present results indicate that regulation of THs, mainly activation by DIO2, plays an important role in developmental neuroplasticity of the song control nuclei. Furthermore, they suggest that active inhibition of TH action by DIO3 after 4 weeks of photostimulation contributes to the stability of HVC necessary for breeding-state song. Whether DIO2 also contributes to song control nucleus growth during seasonal plasticity is yet unclear but is a promising lead for further research. Our data provide essential information on a potentially crucial mechanism in learning-associated neuroplasticity; information that will hopefully help to further elucidate the molecular machinery involved in vertebrate, and especially human, memory processing.