EFFECTS OF THE AROMATASE INHIBITOR R76713 ON SEXUAL-DIFFERENTIATION OF BRAIN AND BEHAVIOR IN ZEBRA FINCHES

Neonatal estradiol injections masculinize both the singing behavior an d some morphological aspects of the song control nuclei in female zebr a finches (Taeniopygia guttata), but there is to date no physiological evidence that estrogens are responsible for the differentiation of so ng and the song control nuclei in male zebra finches during normal ont ogeny. Injections of estrogen into young males demasculinizes their co pulatory behavior, so that it is difficult to understand how the same endocrine environment could simultaneously masculinize song and demasc ulinize copulation. Attempts to block early estrogen action in males h ave resulted in a paradoxical hypermasculinization of several aspects of the song control nuclei. In the present experiment, we tried to eva luate the role played by estrogens during zebra finch differentiation by blocking their secretion with the non-steroidal aromatase inhibitor , R76713 or racemic vorozole. Zebra finches received at 2-3 days post- hatch one Silastic implant filled with R76713 or left empty as a contr ol. Implants were left in place until about 45 days of age and birds w ere then gonadectomized. At the age of 105 days, all birds received a Silastic implant filled with T and, starting two weeks later, they wer e repeatedly tested for singing and copulatory behavior during a perio d of 3 weeks. They were then perfused and their brains were analyzed b y histological methods to measure the volume of the song control nucle i, HVc, RA, area X and MAN. Treatment with R76713 significantly decrea sed (+/- 50%) the number of song bouts produced by the birds but did n ot affect the copulatory behavior of the males. The behavior of female s was not changed by the aromatase inhibitor and, in particular, no in crease in mounting behavior was seen in the treated females. The prese nce of a strong sex dimorphism in the size of all 4 song control nucle i was confirmed but no significant effect of R76713 on these measures could be detected. These data are therefore consistent with the idea t hat estrogens are implicated in the differentiation of singing behavio r in the zebra finch but the small amplitude of the behavioral effect observed and the absence of morphological effect suggest that either o ther factors also play a major role at this level or that higher doses or longer treatments with the aromatase blocker should be tested.