Effect of age and testosterone on autumnal neurogenesis in male European starlings (Sturnus vulgaris).

The male European starling (Sturnus vulgaris) is an open-ended learner that increases its repertoire throughout life. In parallel, the volume of high vocal center (HVC) is larger in older birds than in yearlings. We labeled with the thymidine analog 5'-bromodeoxyuridine (BrdU) the cells that are generated during the fall in the brain of adult males that were 2 or more years old and in yearling males that were treated with exogenous testosterone (T) or kept intact before BrdU administration. In all subjects, the singing rate was recorded and BrdU-labeled cells were quantified in HVC, in proliferative areas of the ventricular zone (VZ) and in auditory regions. BrdU-containing cells were observed in all brain regions investigated. They were significantly more numerous in the VZ of the T-treated yearlings than in any other group. In older birds, a reduced number of labeled cells was specifically observed in the VZ close to the anterior commissure. No group difference was detected in auditory processing areas or in HVC. These data show for the first time a positive influence of T on the production of new cells at the VZ level in a male songbird and a decrease of this process with age. Furthermore, in T-treated birds, a correlation was observed between the HVC volume and the number of differentiated (round) BrdU-positive cell numbers in HVC on the one hand and song rate on another hand supporting the notion that singing activity is causally related to the T-induced growth of this song control nucleus.