Effects of in ovo estradiol benzoate treatments on sexual behavior and size of neurons in the sexually dimorphic medial preoptic nucleus of Japanese quail.

In male Japanese quail, the injection of estradiol benzoate (EB) in the egg causes a complete loss of the testosterone (T)-induced copulatory behavior (behavioral demasculinization). This treatment is only effective if performed before day 12 of incubation. This suggests that embryonic estrogens affect in a permanent way brain structures controlling sexual behavior. The medial preoptic nucleus (POM) is the only sexually dimorphic brain nucleus identified in quail and it is a key center for the action of T on behavior. Recent morphometric studies revealed that the dorsolateral population of neurons in the POM is sensitive to variations of plasma T levels in adult males but not in females. These effects are not observed in the medial part of the POM. This sexually differentiated feature might therefore be organized by the embryonic hormonal environment. Quail embryos of both sexes were treated with EB or received a control injection either on day 9 or on day 14 of egg incubation and the cell size in the dorsolateral and medial POM was evaluated by classical morphometric techniques. EB treatment on day 9 decreased cell size in the dorsolateral POM neurons of males but had no effect in females. No effect was also observed when the treatment was performed on day 14. The medial POM neurons were not affected. The decrease in neuronal size observed in the dorsolateral POM of males treated with EB on day 9 was correlated with their behavioral demasculinization. Some of these males showed a weak sexual behavior during the tests and in parallel had significantly larger neurons.(ABSTRACT TRUNCATED AT 250 WORDS)