Testosterone Induction of Male Typical Song and Associated Neural Plasticity in Adult Female Canaries and European Starlings
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Date
2014-05-05
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Johns Hopkins University
Abstract
We have learned much from the songbird model. As the foremost animal model of vocal learning it has helped to elucidate the intricacies of neural control/coordination of musculature and respiration in vocal production. It has also clearly demonstrated the role of sensory perception and experimentation in vocal-motor sequence learning. The current set of experiments expands the impact of the songbird model by investigating these proximal mechanisms from the vantage point of sexual differentiation and adult neuroplasticity. The female songbird, in these experiments, demonstrates the sheer power of steroids in modifying behavioral output and the neural substrate of social/reproductive behaviors. In treating adult female songbirds of particular species with testosterone (T) one can study the effects of testosterone on sensorimotor learning: that being said, there are many remaining questions. There are three fundamental questions addressed in this thesis; 1) what is the trajectory of vocal development (i.e. sensorimotor learning) in T-induced adult female song and is it different from what occurs in T treated adult male song? 2) Does reproductive state limit the effectiveness of T to induce singing behavior in adult female songbirds? 3) What neural substrates are essential for the induction of song and the recapitulation of sensorimotor learning in adult females? Though male canaries showed had a shorter latency to respond to T-treatment by singing, female canaries did not differ from males in the overall pattern of song learning. However, T masculinizes female canary song only to a certain extent; sexually receptive reproductively active adult female canaries preferred to solicit in response to male T-treated song compared to female T-treated song. Reproductive state modulated the efficacy of T in inducing song behavior in adult female starlings. Lesions targeted to the anterior forebrain pathway increased syllable stereotypy and blocked song learning in T-treated female canary. T-treatment modulated NR2A and NR2B immunoreactivity in the anterior forebrain in association with increases in singing. These findings suggest that though there are elements of song phenotype that cannot be masculinized or de-feminized in adulthood, steroids can activate pathways that are shared by males and females. This activation of latent pathways in females results in a different pattern of sexually differentiated brain and behavior, a pattern that in the future can be used to ask basic questions about the proximal control of learned song behavior.
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Keywords
Songbird, Female, Testosterone, Sex differences, HVC, LMAN, NMDA, Song learning, Song playback