S. Marc Breedlove

Organizational and activational effects of sex steroids on brain and behavior: A reanalysis

The actions of sex steroids on brain and behavior traditionally have been divided into organizational and activational effects. Organizational effects are permanent and occur early in development; activational effects are transient and occur throughout life. Over the past decade, experimental results have accumulated which do not fit such a simple two-process theory. Specifically, the characteristics said to distinguish organizational and activational effects on behavior are sometimes mixed, as when permanent effects occur in adulthood. Attempts to determine whether specific cellular processes are uniquely associated with either organizational or activational effects are unsuccessful. These considerations blur the organizational-activational distinction sufficiently to suggest that a rigid dichotomy is no longer tenable.

Finger-length ratios and sexual orientation

Animal models have indicated that androgenic steroids acting before birth might influence the sexual orientation of adult humans. Here we examine the androgen-sensitive pattern of finger lengths, and find evidence that homosexual women are exposed to more prenatal androgen than heterosexual women are; also, men with more than one older brother, who are more likely than first-born males to be homosexual in adulthood, are exposed to more prenatal androgen than eldest sons. Prenatal androgens may therefore influence adult human sexual orientation in both sexes, and a mothers body appears to ‘remember’ previously carried sons, altering the fetal development of subsequent sons and increasing the likelihood of homosexuality in adulthood.

Sexual differentiation of the vertebrate nervous system

Understanding the mechanisms that give rise to sex differences in the behavior of nonhuman animals may contribute to the understanding of sex differences in humans. In vertebrate model systems, a single factor—the steroid hormone testosterone—accounts for most, and perhaps all, of the known sex differences in neural structure and behavior. Here we review some of the events triggered by testosterone that masculinize the developing and adult nervous system, promote male behaviors and suppress female behaviors. Testosterone often sculpts the developing nervous system by inhibiting or exacerbating cell death and/or by modulating the formation and elimination of synapses. Experience, too, can interact with testosterone to enhance or diminish its effects on the central nervous system. However, more work is needed to uncover the particular cells and specific genes on which testosterone acts to initiate these events.

Sexual Differentiation of the Vertebrate Brain: Principles and Mechanisms☆☆☆

A wide variety of sexual dimorphisms, structural differences between the sexes, have been described in the brains of many vertebrate species, including humans. In animal models of neural sexual dimorphism, gonadal steroid hormones, specifically androgens, play a crucial role in engendering these differences by masculinizing the nervous system of males. Usually, the androgen must act early in life, often during the fetal period to masculinize the nervous system and behavior. However, there are a few examples of androgen, in adulthood, masculinizing both the structure of the nervous system and behavior. In the modal pattern, androgens are required both during development and adulthood to fully masculinize brain structure and behavior. In rodent models of neural sexual dimorphism, it is often the aromatized metabolites of androgen, i.e., estrogens, which interact with estrogen receptors to masculinize the brain, but there is little evidence that aromatized metabolites of androgen play this role in primates, including humans. There are other animal models where androgens themselves masculinize the nervous system through interaction with androgen receptors. In the course of masculinizing the nervous system, steroids can affect a wide variety of cellular mechanisms, including neurogenesis, cell death, cell migration, synapse formation, synapse elimination, and cell differentiation. In animal models, there are no known examples where only a single neural center displays sexual dimorphism. Rather, each case of sexual dimorphism seems to be part of a distributed network of sexually dimorphic neuronal populations which normally interact with each other. Finally, there is ample evidence of sexual dimorphism in the human brain, as sex differences in behavior would require, but there has not yet been any definitive proof that steroids acting early in development directly masculinize the human brain.

Sexually dimorphic motor nucleus in the rat lumbar spinal cord: Response to adult hormone manipulation, absence in androgen-insensitive rats

There is a sexually dimorphic motor nucleus in the fifth and sixth lumbar segments of the rat spinal cord, consisting of motoneurons innervating two striated perineal muscles, the levator ani and the bulbocavernosus. This nucleus, which is diminished or absent in female rats, has been named the spinal nucleus of the bulbocavernosus (SNB)3. We now report that the number of neurons in the SNB of either male or female rats is not altered by adult gonadectomy or treatment with testosterone propionate. However, the size of individual SNB neurons is increased in the presence of androgen in either sex. Genetically male rats with the testicular feminization mutation which results in reduced receptors have a markedly feminine SNB. These results support the hypothesis that the sexually dimorphic nature of the SNB depends on neither the adult hormonal state nor the presence of a Y chromosome, but on the interaction of androgens with their receptors early in development.

Masculinized finger length patterns in human males and females with congenital adrenal hyperplasia

The ratio of the length of the second digit (2D) to the length of the fourth digit (4D) is greater in women than in men. Since androgens are involved in most somatic sex differences and since the sexual dimorphism in 2D:4D is stable from 2 years of age in humans, it was hypothesized that finger length pattern development might be affected by early androgen exposure. Human females with congenital adrenal hyperplasia (CAH) are exposed prenatally to higher than normal levels of adrenal androgens, providing an opportunity to test the effects of early androgen exposure on digit ratios. The 2D:4D was calculated for females with CAH, females without CAH, males with CAH, and males without CAH. Females with CAH had a significantly smaller 2D:4D on the right hand than did females without CAH. Males with CAH had a significantly smaller 2D:4D on the left hand than did males without CAH. A subset of six males with CAH had a significantly smaller 2D:4D on both hands compared with their male relatives without CAH. These results are consistent with the idea that prenatal androgen exposure reduces the 2D:4D and plays a role in the establishment of the sex difference in human finger length patterns. Finger lengths may therefore offer a retrospective marker of perinatal androgen exposure in humans.

Sexual dimorphism and the influence of neonatal androgen in the dorsolateral motor nucleus of the rat lumbar spinal cord.

There is a sexually dimorphic motor nucleus, the spinal nucleus of the bulbocavernosus (SNB) in the fifth and sixth lumbar segments of the rat spinal cord. We now report a second sex difference in the dorsolateral nucleus (DLN) in the ventral horn of the rat lumbar cord, which includes motoneurons innervating the ischiocavernosus muscle, a sexually dimorphic perineal muscle. Adult females possess fewer motoneurons in the DLN, probably because of an absence of neurons innervating the ischiocavernosus muscle, which females lack. The effect of a single dose of testosterone propionate on day 2 of life was confined to a specific rostrocaudal region of the adult DLN in which it partially masculinized the female DLN. Masculinized females have more DLN neurons than control females. The direction of change induced in DLN neuron number by the neonatal hormone treatment is compatible with the hypothesis that androgens are involved with the sexually dimorphic development of the DLN. In another motor nucleus, the retrodorsolateral nucleus, a small sex difference in neuron number was found in one study, but was not replicated in a second experiment.

Minireview: Organizational Hypothesis: Instances of the Fingerpost

There is now compelling evidence that the ratio of the length of the second digit divided by the length of the fourth digit (2D:4D) is affected by prenatal androgens in humans. This ratio is greater in females than males from fetal life through adulthood, correlates with polymorphism in the androgen receptor gene in men, is feminine in XY androgen insensitivity syndrome, and masculinized in congenital adrenal hyperplasia. Using 2D:4D as a correlate, researchers have found evidence that prenatal androgens affect many sexually differentiated human behaviors, including sexual orientation in women (but not in men), attention deficit disorder, autism, eating disorders, aggression, and risk-taking. In each case, lower 2D:4D, indicative of greater prenatal androgen stimulation, is associated with behavior more commonly displayed by males than females. The correlation between 2D:4D and prenatal androgen stimulation is too imperfect to accurately predict the phenotype of a particular individual, even in terms of sex. However, digit ratio is the best available retrospective marker of average differences in prenatal androgen stimulation between groups of people, and/or correlations of prenatal androgen stimulation with particular behaviors and characteristics within a group. Thus digit ratios offer a valid test of the organizational hypothesis that androgens act early in life to masculinize various human behaviors.

The Role of Androgen Receptors in the Masculinization of Brain and Behavior: What we’ve learned from the Testicular Feminization Mutation

Many studies demonstrate that exposure to testicular steroids such as testosterone early in life masculinizes the developing brain, leading to permanent changes in behavior. Traditionally, masculinization of the rodent brain is believed to depend on estrogen receptors (ERs) and not androgen receptors (ARs). According to the aromatization hypothesis, circulating testosterone from the testes is converted locally in the brain by aromatase to estrogens, which then activate ERs to masculinize the brain. However, an emerging body of evidence indicates that the aromatization hypothesis cannot fully account for sex differences in brain morphology and behavior, and that androgens acting on ARs also play a role. The testicular feminization mutation (Tfm) in rodents, which produces a nonfunctional AR protein, provides an excellent model to probe the role of ARs in the development of brain and behavior. Tfm rodent models indicate that ARs are normally involved in the masculinization of many sexually dimorphic brain regions and a variety of behaviors, including sexual behaviors, stress response and cognitive processing. We review the role of ARs in the development of the brain and behavior, with an emphasis on what has been learned from Tfm rodents as well as from related mutations in humans causing complete androgen insensitivity.

Overexpression of wild-type androgen receptor in muscle recapitulates polyglutamine disease

We created transgenic mice that overexpress WT androgen receptor (AR) exclusively in their skeletal muscle fibers. Unexpectedly, these mice display androgen-dependent muscle weakness and early death, show changes in muscle morphology and gene expression consistent with neurogenic atrophy, and exhibit a loss of motor axons. These features reproduce those seen in models of Kennedy disease, a polyglutamine expansion disorder caused by a CAG repeat expansion in the AR gene. These findings demonstrate that toxicity in skeletal muscles is sufficient to cause motoneuron disease and indicate that overexpression of the WT AR can exert toxicity comparable with the polyglutamine expanded protein. This model has two clear implications for Kennedy disease: (i) mechanisms affecting AR gene expression may cause neuromuscular symptoms similar to those of Kennedy disease and (ii) therapeutic approaches targeting skeletal muscle may provide effective treatments for this disease.