Laura L. Carruth

Sex chromosome genes directly affect brain sexual differentiation.

Sex differences in the brain are caused by differences in gonadal secretions: higher levels of testosterone during fetal and neonatal life cause the male brain to develop differently than the female brain. In contrast, genes encoded on the sex chromosomes are not thought to contribute directly to sex differences in brain development, even though male (XY) cells express Y-chromosome genes that are not present in female (XX) cells, and XX cells may have a higher dose of some X-chromosome genes. Using mice in which the genetic sex of the brain (XX versus XY) was independent of gonadal phenotype (testes versus ovaries), we found that XY and XX brain cells differed in phenotype, indicating that a brain cells complement of sex chromosomes may contribute to its sexual differentiation.

Cortisol and Pacific Salmon: A New Look at the Role of Stress Hormones in Olfaction and Home-stream Migration

Abstract Pacific salmon (genus Oncorhynchus) exhibit an interesting and uncommon life-history pattern that combines semelparity, anadromy, and navigation (homing). During smoltification, young salmon imprint on the chemical composition of their natal stream water (the home-stream olfactory bouquet or “HSOB”); they then migrate to the ocean where they spend a few years feeding prior to migrating back to their natal freshwater stream to spawn. Upstream migration is guided by the amazing ability to discriminate between the chemical compositions of different stream waters and thus identify and travel to their home-stream. Pacific salmon demonstrate marked somatic and neural degeneration changes during home-stream migration and at the spawning grounds. The appearance of these pathologies is correlated with a marked elevation in plasma cortisol levels. While the mechanisms of salmonid homing are not completely understood, it is known that adult salmon continuously utilize two of their primary sensory systems, olfaction and vision, during homing. Olfaction is the primary sensory system involved in freshwater homing and “HSOB” recognition, and will be emphasized here. Previously, we proposed that the increase in plasma cortisol during Pacific salmon home-stream migration is adaptive because it enhances the salmons ability to recall the imprinted memory of the “HSOB” (Carruth, 1998; Carruth et al., 2000b). Elevated plasma concentrations of cortisol could prime the hippocampus or other olfactory regions of the brain to recall this memory and, therefore, aid in directing the fish to their natal stream. Thus, specific responses of salmon to stressors could enhance reproductive success.

Does the Segregation of Evolution in Biology Textbooks and Introductory Courses Reinforce Students’ Faulty Mental Models of Biology and Evolution?

The well-established finding that substantial confusion and misconceptions about evolution and natural selection persist after college instruction suggests that these courses neither foster accurate mental models of evolution’s mechanisms nor instill an appreciation of evolution’s centrality to an understanding of the living world. Our essay explores the roles that introductory biology courses and textbooks may play in reinforcing undergraduates’ pre-existing, faulty mental models of the place of evolution in the biological sciences. Our content analyses of the three best-selling introductory biology textbooks for majors revealed the conceptual segregation of evolutionary information. The vast majority of the evolutionary terms and concepts in each book were isolated in sections about evolution and diversity, while remarkably few were employed in other sections of the books. Standardizing the data by number of pages per unit did not alter this pattern. Students may fail to grasp that evolution is the unifying theme of biology because introductory courses and textbooks reinforce such isolation. Two goals are central to resolving this problem: the desegregation of evolution as separate “units” or chapters and the active integration of evolutionary concepts at all levels and across all domains of introductory biology.

Distribution and subcellular localization of glucocorticoid receptor-immunoreactive neurons in the developing and adult male zebra finch brain.

Stress has long lasting effects on physiology, development, behavior, reproductive success and the survival of an individual. These effects are mediated by glucocorticoids, such as corticosterone, via glucocorticoid receptors (GR), however the exact mechanisms underlying these effects are unknown. GR have been widely studied in mammals but little is known about GR in other vertebrate groups, especially songbirds. We investigated the distribution, quantity, and subcellular-localization of GR-immunoreactive (GRir) neurons in the brains of male zebra finches on P10 (post-hatch day 10, song nuclei formed), and in adulthood (post-hatch day 90 or older) using immunohistochemistry. GRir neurons were widely distributed in the brains of male zebra finches including two song nuclei HVC (acronym is a proper name) and RA (nucleus robustus arcopallii) and brain regions including HP (hippocampal formation), BSTl (lateral part of the bed nucleus of the stria terminalis), POM (nucleus preopticus medialis), PVN (nucleus paraventricularis magnocellularis), TeO (optic tectum), S (nucleus of the solitary tract), LoC (Locus coeruleus). Distribution did not vary at the two age points examined, however there were significant differences in staining intensity. Subcellular GR-immunoreactivity patterns were classified as cytoplasmic, nuclear, or both (cytoplasmic and nuclear) and there were significant differences in the overall number of GRir neurons and neurons with both nuclear and cytoplasmic staining in P10 and adult brains. However, there were no significant differences in the percentage of subcellular GR immunoreactivity patterns between P10 and adults. Our study of GRir neuronal distribution in the zebra finch brain may contribute towards understanding of the complex and adverse effects of stress on brain during two different stages of life history.

Sexual responses of the male rat medial preoptic area and medial amygdala to estrogen I: Site specific suppression of estrogen receptor alpha

Male rat copulation is mediated by estrogen-sensitive neurons in the medial preoptic area (MPO) and medial amygdala (MEA); however, the mechanisms through which estradiol (E(2)) acts are not fully understood. We hypothesized that E(2) acts through estrogen receptor α (ERα) in the MPO and MEA to promote male mating behavior. Antisense oligodeoxynucleotides (AS-ODN) complementary to ERα mRNA were bilaterally infused via minipumps into either brain area to block the synthesis of ERα, which we predicted would reduce mating. Western blot analysis and immunocytochemistry revealed a knockdown of ERα expression in each brain region; however, compared to saline controls, males receiving AS-ODN to the MPO showed significant reductions in all components of mating, whereas males receiving AS-ODN to the MEA continued to mate normally. These results suggest that E(2) acts differently in these brain regions to promote the expression of male rat sexual behavior and that ERα in the MPO, but not in the MEA, promotes mating.

The selective estrogen receptor-alpha coactivator, RPL7, and sexual differentiation of the songbird brain

The brain and behavior of the Australian zebra finch (Taeniopygia guttata) are sexually dimorphic. Only males sing courtship songs and the regions of the brain involved in the learning and production of song are significantly larger in males than females. Therefore the zebra finch serves as an excellent model for studying the mechanisms that influence brain sexual differentiation, and the majority of past research on this system has focused on the actions of steroid hormones in the development of these sex differences. Coregulators, such as coactivators and corepressors, are proteins and RNA activators that work by enhancing or depressing the transcriptional activity of the nuclear steroid receptor with which they associate, and thereby modulating the development of sex-specific brain morphologies and behaviors. The actions of these proteins may help elucidate the hormonal mechanisms that underlie song nuclei development. Research described in this review focus on the role of estrogen receptor coactivators in the avian brain; more specifically we will focus on the role of RPL7 (ribosomal protein L7; also known as L7/SPA) on sexual differentiation of the zebra finch song system. Collectively, these studies provide information about the role of steroid receptor coactivators on development of the zebra finch song system as well as on sexual differentiation of brain.

Freshwater Cichlid Crenicara punctulata Is a Protogynous Sequential Hermaphrodite

Abstract Several papers have suggested that the Freshwater Cichlid Crenicara punctulata is capable of protogynous sex change. These studies provide behavioral data and descriptive details of external morphology but lack information about gonadal histology, which is essential for definitive evidence of sequential hermaphroditism. Therefore, this study was designed to determine whether C. punctulata is a sequential hermaphrodite. The present study included a behavioral experiment, an isolation/transformation experiment, as well as a detailed analysis of gonadal structure. The behavioral experiment established social hierarchies in four groups of juvenile female C. punctulata. The hypothesis that the dominant female in each group would be the only individual to develop male secondary sexual characteristics in that group was verified. The isolation experiment tested the hypothesis that female C. punctulata would change sex without the presence of conspecifics. After social isolation, females that were previously dominant among a group of females in a male harem developed male secondary sexual characteristics. Histological analysis revealed that these individuals possessed testes, whereas all dominant females examined possessed mature ovaries. The results from behavioral, isolation, and histological portions of this project strongly suggest that C. punctulata is a protogynous sequential hermaphrodite, at least in captivity.

Distribution and sexually dimorphic expression of steroid receptor coactivator-1 (SRC-1) in the zebra finch brain

Coactivator proteins, such as steroid receptor coactivator-1 (SRC-1) greatly enhance gene expression by amplifying steroid-induced transcription regulated by receptors such as estrogen receptor. These proteins may also play a role in the development of sex differences in central nervous system as well the maintenance of the sexually dimorphic behaviors in adulthood. One well-studied sexually dimorphic behavior is singing in songbirds such as the Australian zebra finch (Taeniopygia guttata). Song learning and production is controlled by the song control system, a collection of sexually dimorphic nuclei found in the avian telencephalon. While the actions of steroid hormones on song nuclei development has been under debate, steroids, such as testosterone, influence singing behavior in adulthood. We hypothesize that the differential expression of coactivators in male and female brains aid in organizing the song nuclei during development and function in adulthood to aid in activating the song control nuclei to induce singing behavior. The distribution of SRC-1-immunoreactive neurons was localized in the brains of male and female zebra finches on the day of hatch (P1) and in adults. In adults SRC-1 immunoreactive cells are found in the four main song control nuclei as well as other steroid sensitive brain regions. We found that SRC-1 is sexually dimorphic in the adult zebra finch telencephalon, suggesting that coactivators may play a role in the maintenance of sexually dimorphic behaviors including singing.

The song remains the same: coactivators and sex differences in the songbird brain.

The majority of songbird species have sexually dimorphic neuronal circuits for song learning and production and these differences are paralleled by sex differences in behavior, with only males singing or singing at a higher rate than females. Therefore songbirds serve as an excellent model for studying the mechanisms that influence the sexually dimorphic development of the brain and behavior. Past research focused on the actions of steroid hormones or their receptors in the development of these sex differences. This review examines the distribution and action of steroid receptor coactivators in the songbird brain; more specifically the actions of RPL7, SRC-1, and CBP on the song control system. Coactivators enhance the transcriptional activity of the nuclear steroid receptors with which they associate, and therefore may play a role in modulating the development of sex differences in the brain and behavior. The actions of these proteins may help elucidate the hormonal mechanisms that underlie song nuclei development and steroid activated singing behavior in adulthood.

Effects of housing condition and early corticosterone treatment on learned features of song in adult male zebra finches.

Early developmental stress can have long-term physiological and behavioral effects on an animal. Developmental stress and early corticosterone (Cort) exposure affect song quality in many songbirds. Early housing condition can act as a stressor and affect the growth of nestlings and adult song, and improvements in housing condition can reverse adverse effects of early stress exposure in rodents. However, little is known about this effect in songbirds. Therefore, we took a novel approach to investigate if housing condition can modify the effects of early Cort exposure on adult song in male zebra finches. We manipulated early housing conditions to include breeding in large communal flight cages (FC; standard housing condition; with mixed-sex and mix-aged birds) versus individual breeding cages (IBC, one male-female pair with small, IBC-S, or large clutches, IBC-L) in post-hatch Cort treated male birds. We found that Cort treated birds from IBC-S have higher overall song learning scores (between tutor and pupil) than from FC but there is no difference between these groups in the No-Cort treated birds. When examining the effects of Cort within each housing condition, overall song learning scores decreased in Cort treated birds from flight cages but increased in birds from IBC-S compared to controls. Likewise, the total number of syllables and syllable types increased significantly in Cort treated birds from IBC-S, but decreased in FC-reared birds though this effect was not statistically significant. These findings suggest that the effects of early Cort treatment on learned features of song depend on housing condition.