Date Published: December 4, 2009
Publisher: Public Library of Science
Author(s): Motoko Mukai, Kirstin Replogle, Jenny Drnevich, Gang Wang, Douglas Wacker, Mark Band, David F. Clayton, John C. Wingfield, Alessandro Bartolomucci. http://doi.org/10.1371/journal.pone.0008182
Abstract: Male song sparrows (Melospiza melodia) are territorial year-round; however, neuroendocrine responses to simulated territorial intrusion (STI) differ between breeding (spring) and non-breeding seasons (autumn). In spring, exposure to STI leads to increases in luteinizing hormone and testosterone, but not in autumn. These observations suggest that there are fundamental differences in the mechanisms driving neuroendocrine responses to STI between seasons. Microarrays, spotted with EST cDNA clones of zebra finch, were used to explore gene expression profiles in the hypothalamus after territorial aggression in two different seasons.
Partial Text: Testosterone (T) has classically been regarded as a major factor in the control of vertebrate aggression, at least in reproductive contexts. However, regulation of aggression may be far more complex as T manipulations have had variable results on aggressive behaviors in different species , , . Moreover, T secretion upon social interaction has been observed in some but not all species (reviewed in ), further adding to the complexity of T’s role in natural aggression. The “challenge hypothesis”  attempts to explain the variable linkage of T and aggression during the breeding season. It suggests that a correlation between the two exists only during social instability, such as during establishment of dominance relationships and/or territorial boundaries, or when animals are “challenged” by a conspecific male for their territories and/or mates. The challenge hypothesis has been supported by studies in over 85 avian species , ,  as well as in over 150 other vertebrates such as fish , reptiles ,  and mammals  including humans .
Song sparrows display similar behavioral responses to STIs in breeding and non-breeding seasons. However, while there is social activation of the hypothalamo-pituitary-gonad axis resulting in an increase on testosterone secretion during breeding season, there is no such response to STIs in the non-breeding season despite very similar behavioral responses. Mechanisms of how increases in LH and T occur during territorial aggression exclusively in the breeding season have not been deciphered to date. Therefore, this study was designed to decipher potential molecular mechanisms underlying neuroendocrine responses during territorial aggression by global gene expression analysis. Our studies detected a large number of cDNAs (727) that are differentially expressed in the hypothalamus according to season or STI. For the most part, these 727 cDNAs represent non-redundant genes, as 555 have now been mapped to defined gene models in the zebra finch genome assembly (Ensembl Release 53, http://www.ensembl.org) and 513 (92% of the mapped genes) of which are unique.