ORGANIZATIONAL EFFECTS OF ESTROGENS ON BRAIN VASOTOCIN AND SEXUAL-BEHAVIOR IN QUAIL

Reproductive behavior is sexually differentiated in quail: The male-ty pical copulatory behavior is never observed in females even after trea tment with high doses of testosterone (T). This sex difference in beha vioral responsiveness to T is organized during the embryonic period by the exposure of female embryo to estrogens. We showed recently that t he sexually dimorphic medial preoptic nucleus (POM), a structure that plays a key role in the activation of male copulatory behavior, is inn ervated by a dense steroid-sensitive network of vasotocin-immunoreacti ve (VT-ir) fibers in male quail. This innervation is almost completely absent in the female POM and is not induced by a chronic treatment wi th T, suggesting that this neurochemical difference could be organizat ional in nature. This idea was tested by injecting fertilized quail eg gs of both sexes on day 9 of incubation with either estradiol benzoate (EB) (25 mu g, a treatment that suppresses the capacity to show copul atory behavior in adulthood) or the aromatase inhibitor R76713 (10 mu g, a treatment that makes adult females behaviorally responsive to T), or with the solvents as a control (C). At 3 weeks posthatch, all subj ects were gonadectomized and later implanted with Silastic capsules fi lled with T. Two weeks later, all birds were perfused and brain sectio ns were processed for VT immunocytochemistry. Despite the similarity o f the adult endocrine conditions of the subjects (all were gonadectomi zed and treated with T Silastic implants providing the same plasma lev el of steroid to all subjects), major qualitative differences were obs erved in the density of VT-ir structures in the POM of the different g roups. Dense immunoreactive structures (fibers and a few cells) were o bserved in the POM of C males but not females; EB males had completely lost this immunoreactivity (and lost the capacity to display copulato ry behavior); and, conversely, R76713 females displayed a male-typical VT-ir system in the nucleus (and also high levels of copulatory behav ior). Similar changes in immunoreactivity were seen in the nucleus of the stria terminalis and in the lateral septum (VT-ir fibers only in t his case) but not in the magnocellular vasotocinergic system. These ne urochemical changes closely parallel the effects of the embryonic trea tments on male copulatory behavior. The vasotocinergic system of the P OM can therefore be considered an accurate marker of the sexual differ entiation of brain circuits mediating this behavior. (C) 1998 John Wil ey & Sons, Inc.