Estradiol and Flutamide Effects on the Song System of Developing Male Zebra Finches

Estradiol (E2) masculinizes the developing song system of female zebra finches (Taeniopygia castanotis) if it is administered in early life, but its effect is blocked with the co-administration of an antiandrogen (Flutamide). The effects of E2 on the developing male song system are not uniform and reports of Flutamide administration in developing male zebra finches differ in their findings. Therefore. this study was conducted to further explore the effects of administering E2 alone, Flutamide (Flut) alone, or the two in combination during early post-hatch development. Brains and testes were examined after day 100. The results showed definite demasculinizing effects of early E2 on the song nucleus HVC (proper name)--its volume and neuron number were markedly reduced. Nonetheless, early E2 hypermasculinized HVC neuronal size. Flut slightly hypermasculinized RA volume (Robust nucleus of the Arcopallium), which replicates one prior study but the absence of additional effects is at odds with others. Early E2 resulted in markedly reducing testes size, which is likely to be a consequence of hijacking endogenous endocrine feedback mechanisms. Arguments are put forward suggesting 1) early E2 action on HVC could be an anachronistic consequence of actions on the genotype of developing male versus females or 2) a disruption of endocrine mechanisms inducing inappropriate hormonal states during development. These possibilities are not mutually exclusive.