Remarkable plasticity is one of the hallmarks of the central nervous system. The vertebrate brain remains plastic throughout the animal’s life, and constantly reshapes itself in response to the animal’s environment, experiences, and physiology. Adult neuroplasticity can be exploited to help patients recover from stroke and traumatic brain injury, while abnormal plasticity is associated with many pathologies, including Alzheimer’s and Parkinson’s diseases. Thus, understanding the mechanisms behind adult neuroplasticity has the potential to help develop better clinical applications to help patients suffering from such disorders. However, adult neuroplasticity processes are rather complex, and can be influenced by many factors, such as stress, seasonal variations, hormonal effects, learning, and memory formation. Two such factors are motor learning and sex hormones. We use adult female canaries as a model system to investigate the processes involved in brain circuit changes in structure and function brought about by sex hormone-induced vocal motor learning experience. Adult female canaries can be induced to produce song, a complex motor skill, by testosterone implantation, which will also trigger the reshaping of the brain regions responsible for song learning and production. We implanted female canaries with testosterone and examined the development of their vocal motor skills over many months, followed by a period when singing practice was abolished by implant removal, and a subsequent period of sex hormone-induced vocal motor skill relearning. We found manifold lasting changes brought about by a first testosterone-induced vocal motor skill learning experience. Singing experience elicited the formation of vocal motor skill memory, as even after long periods when practice was abolished, the rate of vocal motor skill relearning was up to 7x faster than the initial learning. This was accompanied by the optimization of the song premotor circuits, with brain nucleus HVC undergoing a remarkable dendritic spine pruning, posing the excitatory synapses within this nucleus as a likely neural correlate for vocal motor skill memory. Once consolidated, this circuit remained stable and resisted further singing practice and sex hormone-induced changes. Furthermore, a first singing experience also prompted changes in sex hormone responsiveness. Female canaries can only be induced to sing by implantation with DHT, a testosterone metabolite, if they had previous testosterone-induced singing experience. This last finding hints at experienced-induced changes in the activation of hormonal receptor-activated signaling cascades, which might be key to preserve previously optimized circuits. Finally, our findings left open questions that needed detailed large volume investigations of the songbird brain circuits with cell type specificity, which were previously unavailable for songbird brain tissue. Thus, we invested in developing such tools and successfully applied tissue clearing and expansion microscopy in combination with light sheet imaging to songbird brain tissue for the first time. Combined with the use of viral vectors for fluorescent protein expression, these techniques make the exploration of large tissue volumes at high resolution possible, enabling detailed connectomics studies, and can also be applied to examine brain wide gene expression. We thus expect these techniques to help answer the questions left unresolved by the findings in this thesis. With the new findings and tools presented in this we work, I hope to contribute to further the understanding on the important mechanisms controlling motor learning- and sex hormone-induced adult neuroplasticity.publishe
