PHOTOPERIODIC CONDITION MODULATES THE EFFECTS OF TESTOSTERONE ON SONGCONTROL NUCLEI VOLUMES IN MALE EUROPEAN STARLINGS

Songbirds possess a specialized network of brain nuclei mediating song learning and production, the song system. In many species, the nuclei in this circuit are larger in volume in males housed on long photoper iods than in males housed on short photoperiods. Exposure to long days initially leads to increases in circulating levels of testosterone (T ) and it is generally believed that volume changes in the song system are controlled by T-dependent effects of photoperiod. However, a growi ng body of evidence indicates that photoperiod can also have T-indepen dent effects on song system morphology. Previous work in our laborator y suggested that photoperiodic condition may modulate the effects of e xogenous testosterone on the volumes of song control nuclei in male Eu ropean starlings (Sturnus vulgaris). In the present report, we tested the hypothesis that photorefractoriness attenuates the effects of T on the volumes of song control nuclei in starlings. One group of long-da y photorefractory males (16L:8D) was exposed to exogenous T and two gr oups of short-day photosensitive birds (8L: 16D) were either implanted with T or blank (control) capsules. After 8 weeks, T levels were elev ated in both groups of T-implanted birds and were undetectable in cont rol animals. Volumes of the high vocal center were larger in T-implant ed photosensitive birds than in both T-implanted photorefractory and c ontrol photosensitive males, which did not differ. These results sugge st that photorefractoriness renders the song nuclei of starlings less sensitive to stimulatory effects of T. Plasticity in the starling song system appears to require the coordination of the appropriate hormona l milieu with a permissive photoperiodic condition. (C) 1997 Academic Press.