Sexual dimorphism in the neuronal circuits of the quail preoptic and limbic regions

A sexually dimorphic nucleus is located in the preoptic area of Japanese qu ail and plays a key role in the activation of male copulatory behavior. The medial preoptic nucleus (POM) is significantly larger in adult male than i n adult female quail. Its volume is steroid-sensitive in adulthood and cons equently decreases after castration but is restored to normal levels by a t reatment with exogenous testosterone. This volumetric difference appears to result only from a sex difference in the adult hormonal milieu and is not affected by embryonic treatments that permanently modify sexual behavior (n o organizational effects). In contrast, some cytoarchitectonic features of the POM such as the size of neurons in the dorso-lateral part of nucleus ap pear to be irreversibly affected by embryonic steroids. The POM is characte rized by the presence of a wide variety of neurotransmitters, neuropeptides , and receptors and can be specifically identified by the presence of a den se cluster of aromatase-immunoreactive cells, by a high density of neuroten sin-immunoreactive cells and fibers and by a dense vasotocinergic innervati on. Some of these neurochemical markers of the dimorphic nucleus are themse lves modulated by steroids. Many of these neurochemical changes appear to p lay a causal role in the control of male sexual behavior. The quail POM thu s represents an excellent model for the analysis of steroid-induced brain p lasticity in a behaviorally relevant context. Microsc. Res. Tech. 54:364-37 4, 2001. (C) 2001 Wiley-Liss, Inc.