Estrogen accumulation in zebra finch song control nuclei: implications for sexual differentiation and adult activation of song behavior.

In zebra finches the gonadal steroid estradiol (E2) directs the sexual differentiation of neural regions controlling song and synergizes with androgens to stimulate song in adulthood. To identify regions where E2 may act to exert these effects, steroid autoradiographic techniques were used to assess cellular accumulation of 3[H]-E2 or its metabolites within various nuclei of the zebra finch brain. In Experiment 1 we examined brains from juvenile females, still within the critical period for E2's effect on sexual differentiation. In Experiment 2 the pattern and extent of labeling in adult male brains was determined following injection of 3[H]-E2, 3[H]-testosterone, or 3[H]-dihydrotestosterone. The results suggest that, both during development and in adulthood, most song-control nuclei contain few E2-accumulating cells. In contrast, many cells densely labeled by 3[H]-E2 or its metabolites are present in the hypothalamus and in close proximity to one song-control region, the hyperstriatum ventralis pars caudalis (HVc). The distribution of these latter cells overlaps with cells that project to another song-related nucleus, Area X. Thus, in Experiment 3 fluorescent retrograde tracing and steroid autoradiographic techniques were combined to determine if E2-accumulating cells project to Area X in adult males. Although a few retrogradely labeled cells were lightly labeled by 3[H]-E2 or its metabolites, for the most part these appear to be two distinct populations of cells. The sparse accumulation of E2 in the zebra finch song system contrasts with that described in other song birds and has important implications as to the mechanism of E2 action on the developing and mature song system.