Arthur P. Arnold

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.

The genome of a songbird.

The zebra finch is an important model organism in several fields with unique relevance to human neuroscience. Like other songbirds, the zebra finch communicates through learned vocalizations, an ability otherwise documented only in humans and a few other animals and lacking in the chicken—the only bird with a sequenced genome until now. Here we present a structural, functional and comparative analysis of the genome sequence of the zebra finch (Taeniopygia guttata), which is a songbird belonging to the large avian order Passeriformes. We find that the overall structures of the genomes are similar in zebra finch and chicken, but they differ in many intrachromosomal rearrangements, lineage-specific gene family expansions, the number of long-terminal-repeat-based retrotransposons, and mechanisms of sex chromosome dosage compensation. We show that song behaviour engages gene regulatory networks in the zebra finch brain, altering the expression of long non-coding RNAs, microRNAs, transcription factors and their targets. We also show evidence for rapid molecular evolution in the songbird lineage of genes that are regulated during song experience. These results indicate an active involvement of the genome in neural processes underlying vocal communication and identify potential genetic substrates for the evolution and regulation of this behaviour.

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.

A Model System for Study of Sex Chromosome Effects on Sexually Dimorphic Neural and Behavioral Traits

We tested the hypothesis that genes encoded on the sex chromosomes play a direct role in sexual differentiation of brain and behavior. We used mice in which the testis-determining gene (Sry) was moved from the Y chromosome to an autosome (by deletion ofSry from the Y and subsequent insertion of anSry transgene onto an autosome), so that the determination of testis development occurred independently of the complement of X or Y chromosomes. We compared XX and XY mice with ovaries (females) and XX and XY mice with testes (males). These comparisons allowed us to assess the effect of sex chromosome complement (XX vs XY) independent of gonadal status (testes vs ovaries) on sexually dimorphic neural and behavioral phenotypes. The phenotypes included measures of male copulatory behavior, social exploration behavior, and sexually dimorphic neuroanatomical structures in the septum, hypothalamus, and lumbar spinal cord. Most of the sexually dimorphic phenotypes correlated with the presence of ovaries or testes and therefore reflect the hormonal output of the gonads. We found, however, that both male and female mice with XY sex chromosomes were more masculine than XX mice in the density of vasopressin-immunoreactive fibers in the lateral septum. Moreover, two male groups differing only in the form of their Sry gene showed differences in behavior. The results show that sex chromosome genes contribute directly to the development of a sex difference in the brain.

Reframing sexual differentiation of the brain

In the twentieth century, the dominant model of sexual differentiation stated that genetic sex (XX versus XY) causes differentiation of the gonads, which then secrete gonadal hormones that act directly on tissues to induce sex differences in function. This serial model of sexual differentiation was simple, unifying and seductive. Recent evidence, however, indicates that the linear model is incorrect and that sex differences arise in response to diverse sex-specific signals originating from inherent differences in the genome and involve cellular mechanisms that are specific to individual tissues or brain regions. Moreover, sex-specific effects of the environment reciprocally affect biology, sometimes profoundly, and must therefore be integrated into a realistic model of sexual differentiation. A more appropriate model is a parallel-interactive model that encompasses the roles of multiple molecular signals and pathways that differentiate males and females, including synergistic and compensatory interactions among pathways and an important role for the environment.

The organizational-activational hypothesis as the foundation for a unified theory of sexual differentiation of all mammalian tissues.

The 1959 publication of the paper by Phoenix et al. was a major turning point in the study of sexual differentiation of the brain. That study showed that sex differences in behavior, and by extension in the brain, were permanently sexually differentiated by testosterone, a testicular secretion, during an early critical period of development. The study placed the brain together in a class with other major sexually dimorphic tissues (external genitalia and genital tracts), and proposed an integrated hormonal theory of sexual differentiation for all of these non-gonadal tissues. Since 1959, the organizational-activational theory has been amended but survives as a central concept that explains many sex differences in phenotype, in diverse tissues and at all levels of analysis from the molecular to the behavioral. In the last two decades, however, sex differences have been found that are not explained by such gonadal hormonal effects, but rather because of the primary action of genes encoded on the sex chromosomes. To integrate the classic organizational and activational effects with the more recently discovered sex chromosome effects, we propose a unified theory of sexual differentiation that applies to all mammalian tissues.

Sex Differences in the Brain: The Not So Inconvenient Truth

### Introduction In 2001 the Institute of Medicine, a branch of the National Academy of Sciences in the U.S.A., concluded that many aspects of both normal and pathological brain functioning exhibit important yet poorly understood sex differences ([Wizemann and Pardu, 2001][1]). Ten years later, the

Sex chromosomes and brain gender

In birds and mammals, differences in development between the sexes arise from the differential actions of genes that are encoded on the sex chromosomes. These genes are differentially represented in the cells of males and females, and have been selected for sex-specific roles. The brain is a sexually dimorphic organ and is also shaped by sex-specific selection pressures. Genes on the sex chromosomes probably determine the gender (sexually dimorphic phenotype) of the brain in two ways: by acting on the gonads to induce sex differences in levels of gonadal secretions that have sex-specific effects on the brain, and by acting in the brain itself to differentiate XX and XY brain cells.

Hormonal control of a developing neuromuscular system. II. Sensitive periods for the androgen-induced masculinization of the rat spinal nucleus of the bulbocavernosus

The spinal nucleus of the bulbocavernosus (SNB) and its target muscles are reduced or absent in normal female rats (Breedlove, S. M., and A. P. Arnold (1980) Science 210: 564–566). We now report that prenatal treatment of females with testosterone propionate (TP) significantly increases the number of SNB neurons found in adulthood. Dihydrotesterone propionate (DHTP) treatment just after but not before birth also masculinizes the number of SNB neurons in females. SNB soma size is significantly masculinized, i.e., enlarged, by administration of androgen prenatally or as late as 7 to 11 days after birth, even though this late postnatal treatment has no effect on the number of SNB cells. Following TP treatment in adulthood, the androgenized females did not display the postural correlates of male copulatory behavior more often than did control females. From these results we infer the following. (1) Androgens act both before and after birth to influence the sexually dimorphic development of the SNB system. (2) There are different sensitive periods for the masculinization of SNB neuronal number and neuronal size, indicating that these two dimorphic characteristics of the SNB are masculinized by somewhat independent mechanisms. (3) TP and DHTP may act via separate mechanisms to alter the number of SNB neurons. (4) Aromatized metabolites of testosterone are not necessary for masculinization of the SNB system. (5) Virilization of the SNB system does not ensure the masculinization of the traditionally defined measures of male copulatory behavior in rodents.

Dosage compensation is less effective in birds than in mammals

Background In animals with heteromorphic sex chromosomes, dosage compensation of sex-chromosome genes is thought to be critical for species survival. Diverse molecular mechanisms have evolved to effectively balance the expressed dose of X-linked genes between XX and XY animals, and to balance expression of X and autosomal genes. Dosage compensation is not understood in birds, in which females (ZW) and males (ZZ) differ in the number of Z chromosomes. Results Using microarray analysis, we compared the male:female ratio of expression of sets of Z-linked and autosomal genes in two bird species, zebra finch and chicken, and in two mammalian species, mouse and human. Male:female ratios of expression were significantly higher for Z genes than for autosomal genes in several finch and chicken tissues. In contrast, in mouse and human the male:female ratio of expression of X-linked genes is quite similar to that of autosomal genes, indicating effective dosage compensation even in humans, in which a significant percentage of genes escape X-inactivation. Conclusion Birds represent an unprecedented case in which genes on one sex chromosome are expressed on average at constitutively higher levels in one sex compared with the other. Sex-chromosome dosage compensation is surprisingly ineffective in birds, suggesting that some genomes can do without effective sex-specific sex-chromosome dosage compensation mechanisms.