SEASONAL-CHANGES IN THE SIZE OF THE AVIAN SONG CONTROL NUCLEUS HVC DEFINED BY MULTIPLE HISTOLOGICAL MARKERS

Bird song is controlled by a discrete network of brain nuclei. The siz e of several song control nuclei changes seasonally in many seasonally breeding songbird species. Reports of seasonal changes in the size of song nuclei have relied primarily on Nissl stains to define the borde rs of these regions. Recent studies found that the size of the song nu cleus higher vocal center (HVC) in male canaries did not change season ally when its borders were defined by histological markers other than Nissl staining. We used three labels to define the borders of the HVC in male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambel ii): Nissl staining, the distribution of acetylcholinesterase-positive neuropil, and the distribution of neurons projecting to another song nucleus, area X. The HVC was larger in males exposed to a breeding pho toperiod and testosterone concentrations than in males exposed to a no nbreeding photoperiod and testosterone concentrations, regardless of w hich of these three methods was used to define the borders of the HVC. This result suggests that seasonal changes in the Nissl-defined borde rs of the HVC reflect changes in the distribution of physiologically r elevant markers of the nucleus and are not merely artifacts of the Nis sl-staining method. (C) 1997 Wiley-Liss, Inc.