Anatomical specificity of the action of testosterone in relation to the regulation of birdsong and the underlying neuroplasticity

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Johns Hopkins University
One important goal of the field of behavioral neuroscience is to develop theories about the relative importance of different parts of the central and peripheral nervous systems in the modulation of behavior. Using canaries, a well-studied songbird, this thesis investigates the different levels at which testosterone acts to regulate song behavior and the underlying neuroplasticity. Given the integral role hormones play in the modulation of birdsong, the distinct sites at which androgen receptors are distributed throughout the songbird brain and periphery, and the distinct roles specific nuclei play in regulating different features of song, songbirds like canaries are especially amenable to such an investigation. The results of this thesis demonstrate testosterone’s role in the regulation of birdsong and neuroplasticity is pleiotropic: the regulation of specific song features, such as the motivation to sing versus song quality, and the neuroplasticity of the circuitry that regulates birdsong, is dependent on where and how (genomic versus non-genomic-like) in the songbird brain and periphery testosterone acts. For instance, testosterone action at the syrinx, the avian vocal organ, plays a different role in the regulation of birdsong compared to its central substrate, the song control system. Within the song control system, the action of testosterone within the sensorimotor region HVC (acronym is name) plays a different regulatory role of song than the motor region called the robust nucleus of the arcopallium. Many of these song features were regulated by the rapid actions of estrogens, a key product of the aromatization of testosterone in the regulation of birdsong. This suggests that estrogens are key in the regulation of song over very short time scales. Lastly, testosterone drove changes in plasticity in the song control system in a direct (acting within specific sites) as well as in an indirect (by enhancing singing activity) manner. This pleiotropic regulation by testosterone also applies to highly socio-sexually relevant vocal signals including in the songs called trills. These results highlight the complex role played by steroid hormones in the coordination of various suites of behaviors into a functional, adaptive response.
steroid hormones, social behavior, neuroplasticity, birdsong