EXPERIMENTAL-ANALYSIS OF SEXUAL-DIFFERENTIATION OF THE ZEBRA FINCH BRAIN

Classical theories of sexual differentiation of brain and behavior hol d that sex differences in the brain arise because of the action of gon adal steroid hormones. In mammals, testosterone secretion by the teste s stimulates a masculine pattern of neural differentiation, whereas fe minine patterns of development occur in the absence of testicular secr etions. In some bird species, estrogen secreted by the ovary is though t to trigger feminine patterns of neural development, whereas masculin e development occurs in the absence of ovaries. Sexual differentiation of the neural circuit for song in zebra finches is not easily explain ed by these theories. Although female zebra finches can be masculinize d by treatments with estrogen, it has proven difficult to prevent masc uline neural development in genetic males by treating them with inhibi tors of estrogen synthesis. Moreover, when genetic female embryos are treated with inhibitors of estrogen synthesis, they develop significan t amounts of testicular tissue that causes little or no masculinizatio n of the song system. Thus, testicular secretions alone appear to be i nsufficient to cause masculine neural differentiation, and other facto rs need to be invoked. These factors may include ovarian secretions th at inhibit masculine development, or direct genetic (nonhormonal) effe cts on neural differentiation. (C) 1997 Elsevier Science Inc.