Effects of testosterone and photoperiodic condition on song production andvocal control region volumes in adult male dark-eyed juncos (Junco hyemalis)

In seasonally breeding male oscines, song learning and expression are contr olled by brain regions (vocal control regions, VCRs) that exhibit seasonal neural plasticity in adulthood. Several VCRs contain androgen receptors, an d gonadal androgens play important roles in the control of seasonal structu ral and functional changes of VCRs. Recent studies also found that adult VC Rs are influenced by factors other than gonadal hormones, including photope riod, but the relative importance of these factors and their mechanisms of action are poorly understood. To address this issue, we investigated the co ntributions of photoperiod and testicular androgens to the regulation of VC R volumes and to the control of song expression in adult dark-eyed juncos, Junco hyemalis. Exposing castrated (CX) photosensitive males to long days ( LD) enhanced their high vocal center (HVc) volumes compared to those of mal es held on short days (SD). These volumes were not further increased by con current testosterone (T) treatment, revealing a marked and gonadal androgen -independent stimulatory influence of photoperiod on the size of this brain region. HVc sizes were smaller in LD-exposed photorefractory than photosen sitive males irrespective of whether birds were intact or had been castrate d before photoperiodic manipulations, but HVc sizes increased in response t o T treatment in intact photorefractory males. Thus, LD exposure can increa se HVc volumes in the absence of gonadal T, but large volume induction in p hotorefractory males requires elevated plasma T levels. Testosterone treatm ent of So-exposed photosensitive males increased HVc, but not Area X, MAN, or RA volumes. Only T-treated males sang and this treatment given to castra tes was equally effective behaviorally when administered to photosensitive, photostimulated, or photorefractory juncos. This result indicates that the stimulating influence of LD exposure on HVc volumes is insufficient to ind uce song in the absence of elevated plasma T levels. (C) 2001 Academic Pres s.