Rapid Effects of Aggressive Interactions on Aromatase Activity and Oestradiol in Discrete Brain Regions of Wild Male White-Crowned Sparrows

Testosterone is critical for the activation of aggressive behaviours. In many vertebrate species, circulating testosterone levels rapidly increase after aggressive encounters during the early or mid‐breeding season. During the late breeding season, circulating testosterone concentrations did not change in wild male white‐crowned sparrows after an aggressive encounter and, in these animals, changes in local neural metabolism of testosterone might be more important than changes in systemic testosterone levels. Local neural aromatisation of testosterone into 17β‐oestradiol (E2) often mediates the actions of testosterone, and we hypothesised that, in the late breeding season, brain aromatase is rapidly modulated after aggressive interactions, leading to changes in local concentrations of E2. In the present study, wild male white‐crowned sparrows in the late breeding season were exposed to simulated territorial intrusion (STI) (song playback and live decoy) or control (CON) for 30 min. STI significantly increased aggressive behaviours. Using the Palkovits punch technique, 13 brain regions were collected. There was high aromatase activity in several nuclei, although enzymatic activity in the CON and STI groups did not differ in any region. E2 concentrations were much higher in the brain than the plasma. STI did not affect circulating levels of E2 but rapidly reduced E2 concentrations in the hippocampus, ventromedial nucleus of the hypothalamus and bed nucleus of the stria terminalis. Unexpectedly, there were no correlations between aromatase activity and E2 concentrations in the brain, nor were aromatase activity or brain E2 correlated with aggressive behaviour or plasma hormone levels. This is one of the first studies to measure E2 in microdissected brain regions, and the first study to do so in free‐ranging animals. These data demonstrate that social interactions have rapid effects on local E2 concentrations in specific brain regions.