2 HISTOLOGICAL MARKERS REVEAL A SIMILAR PHOTOPERIODIC DIFFERENCE IN THE VOLUME OF THE HIGH VOCAL CENTER IN MALE EUROPEAN STARLINGS

Most studies of seasonal changes in the avian song control system have used Nissl stains to characterize the nuclei. More recent work has in dicated that changes in nucleus volume evident in Nissl-stained tissue are not always apparent when investigated with other histochemical cr iteria. In this experiment, we used two different markers (Nissl stain and az-adrenergic receptor autoradiography) to characterize changes i n the song system of European starlings (Sturnus vulgaris). Fluctuatin g levels of circulating testosterone (T) appear to be causally related to seasonal changes in the song system. Therefore, we used photoperio d manipulations to place male starlings into different physiological c onditions. Photosensitive male starlings were placed on 11L:13D or 16L :8D photoperiods for at least 5 months. Birds on 11L:13D have enlarged gonads and circulating T. In contrast, starlings maintained on 16L:8D initially show marked gonadal growth. However, after about 6-8 weeks the birds are photorefractory (i.e., the gonads are regressed and T fa lls to undetectable levels). The volume of the high vocal center (HVC) was 44% larger in the 11L:13D than in 16L:8D birds in Nissl-stained t issue. The density of alpha(2)-adrenergic receptors as determined by i n vitro receptor autoradiography with [H-3]p-amino-clonidine (PAC) is higher in HVC than in the surrounding neostriatum, clearly delineating the boundaries of the nucleus. We reconstructed the volume of HNC usi ng PAC autoradiography on adjacent sections. The results were identica l to those from the Nissl-stained tissue. Thus, two histochemical mark ers indicate a photoperiodic difference in HVC volume of male starling s. (C) 1995 Wiley-Liss, Inc.