Abstract
Photoperiod and hormonal cues drive dramatic seasonal changes in structure and function of the avian song control system. Little is known, however, about the patterns of gene expression associated with seasonal changes. Here we address this issue by altering the hormonal and photoperiodic conditions in seasonally-breeding Gambel's white-crowned sparrows and extracting RNA from the telencephalic song control nuclei HVC and RA across multiple time points that capture different stages of growth and regression. We chose HVC and RA because while both nuclei change in volume across seasons, the cellular mechanisms underlying these changes differ. We thus hypothesized that different genes would be expressed between HVC and RA. We tested this by using the extracted RNA to perform a cDNA microarray hybridization developed by the SoNG initiative. We then validated these results using qRT-PCR. We found that 363 genes varied by more than 1.5 fold (>log2 0.585) in expression in HVC and/or RA. Supporting our hypothesis, only 59 of these 363 genes were found to vary in both nuclei, while 132 gene expression changes were HVC specific and 172 were RA specific. We then assigned many of these genes to functional categories relevant to the different mechanisms underlying seasonal change in HVC and RA, including neurogenesis, apoptosis, cell growth, dendrite arborization and axonal growth, angiogenesis, endocrinology, growth factors, and electrophysiology. This revealed categorical differences in the kinds of genes regulated in HVC and RA. These results show that different molecular programs underlie seasonal changes in HVC and RA, and that gene expression is time specific across different reproductive conditions. Our results provide insights into the complex molecular pathways that underlie adult neural plasticity.
Highlights
The birth and death of neurons and the growth and retraction of their axonal and dendritic arbors are critical features that mediate adult plasticity of the vertebrate brain
In order to determine which genes vary in expression in HVC and RA across breeding conditions, we placed 36 male whitecrowned sparrows (Zonotrichia leucophrys gambelii) under short day (SD) photoperiod to ensure they were in a photosensitive, nonbreeding state
The mRNA from each nucleus from each bird was hybridized on a separate array (71 arrays in total; we extracted an insufficient amount of mRNA from one HVC)
Summary
The birth and death of neurons and the growth and retraction of their axonal and dendritic arbors are critical features that mediate adult plasticity of the vertebrate brain. These processes, accompanied by changes in the electrophysiological properties of neurons, permit the brain to adapt to both short term and long term environmental changes, allowing the organism to survive and successfully reproduce. Breeding songbirds show dramatic plasticity in adult brain and behavior between the breeding and nonbreeding seasons [1] In these species, photoperiod-induced changes in circulating levels of steroid sex hormones induce changes in the neural structure [2], neuron number [3], and electrophysiological properties [4] of the brain regions that control behaviors related to reproduction. This includes song, a learned behavior used for defending territories and attracting females [5], and the song control system, a network of discrete brain nuclei that control song learning and production [6]
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