It is well known that the class of steroid hormones known as estrogens have powerful effects on organs related to reproduction such as the uterus and the breast. What is less well known is that estrogens also profoundly modulate brain function and behavior. Estrogens, such as estradiol, can occur in brain as the result of ovarian secretion of the hormone into the blood that then finds its way to the brain. In male vertebrates, the testes secrete androgens, such as testosterone, into the blood and this class of steroid hormones can be converted into estrogens in the brain via the action of the enzyme aromatase which is expressed in the male brain in many species. Finally estradiol can be synthesized de novo from cholesterol as it has been shown in a variety of species that all the enzymes required to synthesize estrogens are expressed in the brain. This book collects chapters by experts in the field that considers, how estradiol is synthesized in the brain and what its effects are on a variety of behaviors. Special attention is paid to the enzyme aromatase that is distributed in discrete regions of the brain and is highly regulated in a sex specific and seasonal specific manner. Recently it has become clear that estrogens can act in the brain at two very different time scales, one is rather long lasting (days to weeks) and involves the modulation of gene transcription by the hormone-receptor complex. A second mode of action is much quicker and involves the action of estrogens on cell membranes that can result in effects on second messenger systems and ultimately behavior within minutes. Thus this book highlights novel views of estrogen action that are still under-appreciated namely that estrogens have significant effects on the male brain and that they can act on two very different times scales. This volume will be of interest to both basic researchers and clinicians interested in the action of estrogens.

In this fascinating book, Jacques Balthazart presents a simple description of the biological mechanisms that are involved in the determination of sexual orientation in animals and also presumably in humans. Using scientific studies published over the last few decades, he argues that sexual orientation, both homosexual and heterosexual, is under the control of embryonic endocrine and genetic phenomena in which there is little room for individual choice. The author begins with animal studies of the hormonal and neural mechanisms that control the so-called instinctive behaviors and analyzes how this animal work may potentially apply to humans. The book does not focus exclusively on homosexuality, however. Instead, the book acts as a broader guide to the biological basis of sexual orientation, and also discusses important gender differences that may influence sexual orientation. While firmly grounded in the scientific literature, this text is developed for a broader audience and will be of interest to psychologists, researchers, students, and anyone interested in the biological factors that determine our sexuality.

Understanding the brain mechanisms which underlie behavior is one of the most challenging tasks of modern biology. The study of these mechanisms can be approached using a variety of biological systems as models, depending essentially on the type of question being asked and the technical approach which is considered. In vertebrates, the study of brain neurochemistry in relation to behavior e, xpression has made tremendous progress during the last two decades. In particular, much at tention has been devoted to the effect of steroid hormones on brain structure and activity in connection with social and mainly reproductive behavior. This book exemplifies some of the major trends in the field. I did not attempt to cover exhaustively all the work that has been done in this area but rather to present a series of selected reviews on the molec ular and cellular brain mechanisms most directly related to social behavior. This selection obviously reflects my own interests but also, I believe, highlights those areas of research in which important progress has been made in recent years. A number of brain biochemical or cellular mechanisms which could be related to behavior, but have not been formally demonstrated to be so, are consequently ignored. The focus here is on social behavior and thus steroid-dependent processes are covered in priority. This choice of course leads us to ignore major trends in brain and behavior research but this is, I hope, clearly reflected in the title of the volume."

Sexual orientation (homo- vs. heterosexuality) is one of many sex differences observed in humans. Sex differences can result from differential postnatal experiences (interaction with parents, environment) or from biological factors (hormones and genes) acting pre- or postnatally. The first option is often favored to explain sexual orientation although it is supported by little experimental evidence. In contrast, many sexually differentiated behaviors are organized during early life by an irreversible action of sex steroids. In particular, the preference for a male or female sex partner is largely determined in rodents by embryonic exposure to sex steroids. The early action of these steroids also seems to affect sexual orientation in humans. Indeed, clinical conditions associated with major endocrine changes during embryonic life often result in an increased incidence of homosexuality. Furthermore, multiple sexually differentiated behavioral, physiological, or even morphological traits that are known to be organized by prenatal steroids, at least in animals, are significantly different in homo- and heterosexual populations. Thus prenatal endocrine (or genetic) factors seem to influence significantly human sexual orientation even if a large fraction of the variance remains unexplained to date. The possible interaction between biological factors acting prenatally and postnatal social influences remains to be investigated.