The sexually dimorphic medial preoptic nucleus of quail: a key brain area mediating steroid action on male sexual behavior.

About 10 years ago, a sexually differentiated nucleus was identified in the preoptic area (POA) of the Japanese quail in the course of studies analyzing the dimorphic mechanisms involved in the activation of sexual behavior. In this species, males exposed to testosterone copulate while females never show this masculine behavior. The present paper reviews anatomical, neurochemical, and functional data that have been collected since that time about the quail dimorphic nucleus. The medial preoptic nucleus (POM) is significantly larger in adult male than in adult female quail. Its volume is also steroid-sensitive in adulthood: it decreases when circulating levels of testosterone are low (castration, exposure to short-days) and it increases when testosterone levels are high (treatment with testosterone, exposure to long-days). The POM is a necessary and sufficient site of steroid action for the activation of male copulatory behavior. The volumetric difference of the POM results from a difference in the adult hormonal milieu of males and females (activational effect) and is not affected by embryonic treatments that permanently modify sexual behavior (no organizational effects on POM). In contrast, the size of neurons in the dorsolateral part of POM appears to be irreversibly affected by embryonic steroids and this feature is therefore a better correlate of the behavioral sex difference. The POM is characterized by the presence of a wide variety of neurotransmitters, neuropeptides, and receptors. It can, in addition, be specifically distinguished from the surrounding POA by the presence of aromatase-immunoreactive cells, by a high density of alpha 2-adrenergic receptors, and by a dense vasotocinergic innervation. Some of these neurochemical markers of the dimorphic nucleus are themselves modulated by steroids. In particular, the aromatase-immunoreactive cells of the lateral POM appear to be a key target for steroids in the activation of male copulatory behavior. The POM is bidirectionally connected to many brain areas. It receives inputs from a variety of sensory areas and from a number of regulatory areas (e.g., catecholaminergic cell groups). This nucleus also sends outputs to "neurovegetative" centers and to brain regions directly connected to the motor pathways. These connections fully support the role of the POM as an integrative center for the control of male sexual behavior. The available data indicate that there is a high degree of steroid-induced neuronal plasticity in the POM, including changes in neuronal function, in protein synthesis, and in specific inputs. These phenomena can easily be studied in the POM because they are of a large magnitude, they are localized in a specific brain site, and they develop rapidly after exposure to steroids. They are also directly related to a clear functional output, the activation of male sexual behavior. The quail POM therefore constitutes an exceptional model for the analysis of steroid-induced brain plasticity in a functionally relevant context.