The influence of melatonin on birdsong and its underlying neuronal correlates : integrating technical advances to address melatonin's role as an endocrine switch

Abstract

Studying how the nervous system mediates behavior is a central theme of behavioral neurobiology. Investigating the neuronal correlates of birdsong is especially attractive because the ethological aspects as well as the organization of the songbird’s brain are already exceptionally well understood. Particularly fascinating is the process of learning and memorizing birdsong as well as the maintenance of this memory, not least due to its similarities to human language. Sleep seems to play an important role for offline processes involved in song learning in juveniles as well as memory consolidation and song maintenance in adult birds, but the exact processes relevant for switching between motor output during the day and memory consolidation and neuronal rehearsal processes during night are yet to be described. Several facts hint towards melatonin as an important factor in this context. In zebra finches, juveniles have difficulties adjusting their plastic song to their tutor’s song and crystallized song in adult birds deteriorates in absence of natural melatonin production. Furthermore, the presence of melatonin binding sites expressed in several nuclei of the song control system, a neuronal network responsible for learning and producing birdsong, hints towards the importance of melatonin for this system. To investigate effects of melatonin on neuronal processes in the song control system disentangled from the confounding factors of sleep was the main goal of this thesis. By adapting and combining available methods to manipulate natural melatonin levels of the zebra finch (chapter 1) we prepared the ground for focusing on the effects of melatonin on neuronal activity in the song control system. As the sole cortical output of the song system and an important connection between auditory, motor and learning pathways, the nucleus robustus of the arcopallium (RA) provided an ideal target area to record neuronal activity underlying birdsong. By utilizing a wireless recording technique (chapter 2) we were able to relate neuronal activity in RA to different aspects of vocal communication in freely behaving groups of zebra finches and to describe the involvement of the song system 12 not only in song production but also in communication via unlearned vocalizations. Finally, by recording single neurons in RA for several consecutive days and at the same time manipulating melatonin levels (chapter 3) we were able to show circadian changes in the activity of single neurons and to describe the influence melatonin might have on this system. Taken together, we suggest melatonin as an endocrine switch directly influencing crucial parts of the song control system and therefore being involved in the regulation of neuronal activity underlying birdsong.