Abstract
The song-control system is a network of discrete nuclei in the songbird brain that controls the production and learning of birdsong and exhibits some of the best-studied neuroplasticity found in the adult brain. Photoperiodic growth of the song-control system during the breeding season is driven, at least in part, by the gonadal steroid testosterone. When acting on neural tissue, however, testosterone can be metabolized into 5α-dihydrotestosterone (DHT) or 17β-estradiol (E2), which activate different hormonal signaling pathways. By treating adult starlings with both testosterone metabolites and metabolite antagonists, we attempted to isolate the effects of androgen and estrogen treatment on neuroplasticity during photostimulation in male and female European starlings (Sturnus vulgaris). Photostimulation resulted in a large HVC volume typical of the breeding season in all treatments independent of hormone treatment. E2 had additional effects on HVC growth by reducing neuron density and enhancing early survival of new neurons recruited to HVC in females but did not significantly affect HVC volume. Conversely, DHT reduced the migration of new neurons, assessed by the expression of doublecortin, to HVC. DHT also increased syrinx mass and maintained RA (robust nucleus of the arcopallium) cytoarchitecture in the presence of aromatase inhibitors. In addition, we document the first evidence of sex-specific neuroplastic responses of the song-control system to androgens and estrogens. These findings suggest that the contributions of DHT and E2 signaling in songbird neuroplasticity may be regulated by photoperiod and that future studies should account for species and sex differences in the brain.
Highlights
Some of the most extreme adult neuroplasticity in vertebrates occurs in the song-control system of songbirds, a circuit of discrete nuclei underlying birdsong production and learning
Ethics Statement All procedures complied with guidelines set out by the Canadian Council on Animal Care (CCAC), and and followed Guidelines to the Use of Wild Birds in Research [21]
We found that photostimulation-induced HVC growth was not affected by treatment with, or antagonism of DHT or E2 in adult male and female European starlings
Summary
Some of the most extreme adult neuroplasticity in vertebrates occurs in the song-control system of songbirds, a circuit of discrete nuclei underlying birdsong production and learning. Vernal increases in photoperiod during the breeding season induce the volumetric growth of song-control system nuclei including HVC (used as a proper name), the robust nucleus of the arcopallium (RA), and Area X. Changes in neuron size, neuron density, synaptogenesis, angiogenesis, neuron turnover and incorporation, and glia density produce larger song-control system nuclei volumes in the breeding season [2,3,4]. These anatomical changes parallel seasonal changes in song [5] and electrophysiology [6]. The magnitude and behavioral correlates of neuroplasticity in the song-control system make songbirds an ideal model for studying the physiological control of adult neuroplasticity
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