Effect of aggressive interactions on aromatase activity in discrete brain regions in wild male white-crowned sparrows

Testosterone (T) is a critical endocrine factor involved in the activation of aggressive behaviors. In many vertebrate species, circulating T levels rapidly increase after aggressive encounters during the breeding season. In contrast, we recently showed that circulating T concentrations did not change in white-crowned sparrows in the late breeding season after simulated territorial intrusions. We suggested that changes in local metabolism of T might be more important than changes in systemic T levels. Neural aromatization of T into 17-estradiol (E2) often mediates the physiological and behavioral actions of T. In vertebrates, aromatase is expressed in several discrete brain regions. We hypothesized that in the late breeding season, brain aromatase is rapidly modulated after aggressive interaction, leading to changes in local concentrations of E2. Wild male white-crowned sparrows were exposed to simulated territorial intrusion with song playback and decoy (STI) or control (CON) for 30 min. STI significantly increased aggressive behaviors. Birds were then caught, rapidly bled and sacrificed. Brains were collected and rapidly frozen on dry ice. We used 0.9 mm diameter punches from 300 μm coronal sections to isolate 13 different brain nuclei. Aromatase activity was analyzed in punches from the left side of the brain, while E2 was analyzed in punches from the right side of the brain. Aromatase activity was quantified by measuring the release of tritiated water during aromatization of [1-3H]-androstenedione. As expected, aromatase activity was high in the medial preoptic area, ventromedial nucleus of the hypothalamus, hippocampus, bed nucleus of the stria terminalis, nucleus taeniae of the amygdala, and caudomedial nidopallium. Aromatase activity was low in the medial magnocellular nucleus of anterior nidopallium, HVC, Area X, nucleus robustus of the arcopallium, optic lobes, periaqueductal gray and cerebellum. Aromatase activity was not different between the STI and CON groups in any region. There were no significant correlations between aromatase activity and aggressive behaviors or endocrine measures (plasma T, progesterone, corticosterone and corticosteroid binding globulin). These data provide no evidence for rapid modulation of brain aromatase activity following aggressive interactions. It is however possible that aromatase activity is more rapidly modulated (e.g. within 5 min) and these changes were not observed in our 30 min paradigm. We are currently investigating whether local E2 is affected by aggressive interactions