Local intracerebral implants of estrogen masculinize some aspects of the zebra finch song system.

The song system of zebra finches is sexually dimorphic: the volumes of the song control nuclei and the neurons within these nuclei are larger in males. The song system of hatchling female zebra finches is masculinized by systemic treatment with estrogen. We investigated the locus of this estrogen action by using microimplants of estradiol benzoate (EB). We implanted female zebra finch nestlings 10-13 days old with Silastic pellets containing approximately 2 micrograms EB at one of several sites: near the higher vocal center (HVC), in the brain distant from HVC, or in the periphery either under the skin of the breast or in the peritoneal cavity. Controls were either unimplanted or implanted near HVC with Silastic pellets without hormone. The brains were fixed by perfusion at 60 days, and the volumes of the song control regions as well as the sizes of individual neurons were measured. Neurons in HVC were larger (more masculine) in the HVC-implanted group than in the other groups, which did not differ among themselves. The size of neurons in the robust nucleus of the archistriatum (RA) and the lateral magnocellular nucleus of the neostriatum (lMAN) were inversely correlated with the distance of the EB pellet to HVC; neurons in RA and lMAN were larger when the EB pellets were closer to HVC. This result suggests that implants near HVC were at or near a site of estrogen action. To our knowledge, this is the first demonstration that localized brain implants of estrogen cause morphological masculinization in any species.