EARLY ADMINISTRATION OF 17-BETA-ESTRADIOL PARTIALLY MASCULINIZES SONGCONTROL REGIONS AND ALPHA(2)-ADRENERGIC RECEPTOR DISTRIBUTION IN EUROPEAN STARLINGS (STURNUS-VULGARIS)

The vocal control system in many songbird species is a sexually dimorp hic neural circuit that mediates learning and production of song. The mechanism by which this system is sexually differentiated has been inv estigated in only one species, the zebra finch (Taeniopygia guttata). Estradiol may be involved in the sexual differentiation of this system , as female zebra finches treated with estradiol as nestlings develop a male-like song system; however, blocking estradiol action in embryon ic and nestling male zebra finches does not demasculinize the song sys tem. Therefore, the role of estradiol in song system development is un clear. The role of estradiol in song system sexual differentiation was assessed in European starlings (Sturnus vulgaris). This species is of potential interest because it is less extreme in the degree of sexual dimorphism of the song system and song behavior than zebra finches. W hile in the field, starling nestlings were implanted with 500 mu g of estradiol at 3 days of age. These birds were brought into the laborato ry at Day 11 and hand-reared. In females, estradiol produces significa nt increases in the volumes of song control regions defined by Nissl s tain, as well as by autoradiography for alpha(2)-adrenergic receptors; however, these estradiol-treated females have song systems that more closely resemble those of control females than control males. Estradio l-treated males exhibit significant hypermasculinization at 210 days o f age, but this effect is transient and hypermasculinization is no lon ger evident at Day 345. The role of estradiol in sexual differentiatio n of the neural circuit mediating song behavior remains enigmatic. (C) 1996 Academic Press.