Correlation between the sexually dimorphic aromatase of the preoptic area and sexual behavior in quail: effects of neonatal manipulations of the hormonal milieu

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Abstract
The aromatase of the preoptic area is significantly more active in males than in females. This sex dimorphism in enzyme activity is still found in birds that have been gonadectomized and treated with a same dose of testosterone. This suggests that the sex difference is not the result of a differential activation by the adult hormonal environment but rather is organized neonatally by steroid hormones. As the central aromatization of testosterone is a limiting step in the activation of copulatory behavior by testosterone, the lower aromatase activity in the preoptic area of females might be responsible, at least in part, for their lower sensitivity to the activating effects of testosterone on behavior. Three experiments were carried out to determine whether early manipulations of the hormonal environment, which are known to differentiate sexual behavior, also affect in a permanent way the aromatase activity in the preoptic area. Injection of estradiol benzoate into male embryos on day 9 of incubation decreased the preoptic aromatase activity in parallel to its demasculinizing effect on behavior. Unexpectedly the same treatment tended to increase enzyme activity in females so that the physiological relevance of the observed enzymatic change remains questionable. In two independent experiments, we confirmed that neonatal ovariectomy of female quail interferes with their behavioral differentiation. Females gonadectomized at 4 days post-hatch showed significantly more male-type sexual behavior as adult in response to testosterone than females gonadectomized at the age of S weeks. These experiments also confirmed that the preoptic aromatase activity is higher in males than in females but no evidence for an effect of the age of gonadectomy on the enzyme activity could be obtained. The sex difference and experimental modifications observed in the aromatase activity of the preoptic area were not seen in the posterior hypothalamus demonstrating that these effects are specific. The mechanisms controlling the sex difference in aromatase activity are discussed. The difference might be organized by the action of embryonic steroids as suggested by the changes observed in males injected with estradiol benzoate in egg. Alternatively, activational mechanisms cannot be ruled out at present. In one experiment, the activity of the preoptic aromatase was positively correlated with the sexual activity of the birds. In addition, the enzyme activity was found to be numerically (although not statistically) higher in males that had been exposed to females than in birds kept in social isolation. The differential interaction with the stimulus females should be studied in detail as this might be the cause rather than the consequence of the enzymatic dimorphism.