THE SEXUALLY DIMORPHIC MEDIAL PREOPTIC NUCLEUS OF QUAIL - A KEY BRAINAREA MEDIATING STEROID ACTION ON MALE SEXUAL-BEHAVIOR

About 10 years ago, a sexually differentiated nucleus was identified i n the preoptic area (POA) of the Japanese quail in the course of studi es analyzing the dimorphic mechanisms involved in the activation of se xual behavior. In this species, males exposed to testosterone copulate while females never show this masculine behavior. The present paper r eviews anatomical, neurochemical, and functional data that have been c ollected 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 (castrat ion, exposure to short-days) and it increases when testosterone levels are high (treatment with testosterone, exposure to long-days). The PO M is a necessary and sufficient site of steroid action for the activat ion of male copulatory behavior. The volumetric difference of the POM results from a difference in the adult hormonal milieu of males and fe males (activational effect) and is not affected by embryonic treatment s that permanently modify sexual behavior (no organizational effects o n POM). In contrast, the size of neurons in the dorsolateral part of P OM appears to be irreversibly affected by embryonic steroids and this feature is therefore a better correlate of the behavioral sex differen ce. The POM is characterized by the presence of a wide variety of neur otransmitters, neuropeptides, and receptors. It can, in addition, be s pecifically distinguished from the surrounding POA by the presence of aromatase-immunoreactive cells, by a high density of az-adrenergic rec eptors, and by a dense vasotocinergic innervation. Some of these neuro chemical markers of the dimorphic nucleus are themselves modulated by steroids. In particular, the aromatase-immunoreactive cells of the lat eral POM appear to be a key target for steroids in the activation of m ale copulatory behavior. The POM is bidirectionally connected to many brain areas. It receives inputs from a variety of sensory areas and fr om 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 connect ions fully support the role of the POM as an integrative center for th e control of male sexual behavior. The available data indicate that th ere 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 bec ause they are of a large magnitude, they are localized in a specific b rain site, and they develop rapidly after exposure to steroids. They a re also directly related to a clear functional output, the activation of male sexual behavior. The quail POM therefore constitutes an except ional model for the analysis of steroid-induced brain plasticity in a functionally relevant context. (C) 1996 Academic Press, Inc.