ROLES OF PHOTOPERIOD AND TESTOSTERONE IN SEASONAL PLASTICITY OF THE AVIAN SONG CONTROL-SYSTEM

The song control nuclei of songbirds undergo pronounced seasonal chang es in size and neuronal attributes, The mechanisms by which seasonal c hanges in environmental variables such as photoperiod mediate seasonal changes in these brain regions are not known, Manipulations of photop eriod and/or testosterone in captive songbirds induce seasonal changes in the size of song nuclei comparable to those observed in wild songb irds, It is unclear, however, whether the effects of photoperiod on th e song nuclei are mediated by testosterone or by steroid-independent m echanisms. We independently manipulated photoperiod and testosterone i n castrated male Gambel's white-crowned sparrows (Zonotrichia leucophr ys gambelii) to determine the contributions of steroid-dependent and - independent actions of photoperiod to seasonal changes in the size and neuronal attributes of song nuclei, Testosterone implants increased t he size of several song nuclei, regardless of photoperiod, Photoperiod exerted small but significant steroid-independent effects on the volu me of the higher vocal center and the size of neurons in the robust nu cleus of the archistriatum, Photoperiod also modulated the effect of t estosterone on the size of area X; testosterone treatment had a more p ronounced effect on the size of area X on short days than on long days , These results suggest that although testosterone is the primary fact or mediating seasonal changes in neural attributes of the song nuclei, photoperiod may act via mechanisms that are independent of steroid le vels to supplement or modulate the actions of testosterone. (C) 1997 J ohn Wiley & Sons, Inc.