Absence of postnatal testosterone fails to demasculinize the male rat's corpus callosum

The effects of puberty and sex on adolescent white matter development: A systematic review

Adolescence, the transition between childhood and adulthood, is characterized by rapid brain development in white matter (WM) that is attributed in part to rising levels in adrenal and gonadal hormones. The extent to which pubertal hormones and related neuroendocrine processes explain sex differences in WM during this period is unclear. In this systematic review, we sought to examine whether there are consistent associations between hormonal changes and morphological and microstructural properties of WM across species and whether these effects are sex-specific. We identified 90 (75 human, 15 non-human) studies that met inclusion criteria for our analyses. While studies in human adolescents show notable heterogeneity, results broadly demonstrate that increases in gonadal hormones across pubertal development are associated with macro- and microstructural changes in WM tracts that are consistent with the sex differences found in non-human animals, particularly in the corpus callosum. We discuss limitations of the current state of the science and recommend important future directions for investigators in the field to consider in order to advance our understanding of the neuroscience of puberty and to promote forward and backward translation across model organisms.

Trajectories and phenotypes with estrogen exposures across the lifespan: What does Goldilocks have to do with it?

This article is part of a Special Issue "Estradiol and cognition". Estrogens impact the organization and activation of the mammalian brain in both sexes, with sex-specific critical windows. Throughout the female lifespan estrogens activate brain substrates previously organized by estrogens, and estrogens can induce non-transient brain and behavior changes into adulthood. Therefore, from early life through the transition to reproductive senescence and beyond, estrogens are potent modulators of the brain and behavior. Organizational, reorganizational, and activational hormone events likely impact the trajectory of brain profiles during aging. A "brain profile," or quantitative brain measurement for research purposes, is typically a snapshot in time, but in life a brain profile is anything but static--it is in flux, variable, and dynamic. Akin to this, the only thing continuous and consistent about hormone exposures across a female's lifespan is that they are noncontinuous and inconsistent, building and rebuilding on past exposures to create a present brain and behavioral landscape. Thus, hormone variation is especially rich in females, and is likely the destiny for maximal responsiveness in the female brain. The magnitude and direction of estrogenic effects on the brain and its functions depend on a myriad of factors; a "Goldilocks" phenomenon exists for estrogens, whereby if the timing, dose, and regimen for an individual are just right, markedly efficacious effects present. Data indicate that exogenously-administered estrogens can bestow beneficial cognitive effects in some circumstances, especially when initiated in a window of opportunity such as the menopause transition. Could it be that the age-related reduction in efficacy of estrogens reflects the closure of a late-in-life critical window occurring around the menopause transition? Information from classic and contemporary works studying organizational/activational estrogen actions, in combination with acknowledging the tendency for maximal responsiveness to cyclicity, will elucidate ways to extend sensitivity and efficacy into post-menopause.

The intersection of neurotoxicology and endocrine disruption

Endocrine disruption, the guiding theme of the 27th International Neurotoxicology Conference, merged into the neurotoxicology agenda largely because hormones help steer the process of brain development. Although the disruption motif first attracted public health attention because of reproductive anomalies in both wildlife and humans, the neurobehavioral implications had been planted decades earlier. They stemmed from the principle that sex differences in behavior are primarily the outcomes of differences in how the brain is sexually differentiated during early development by gonadal hormones (the Organizational Hypothesis). We also now understand that environmental chemicals are capable of altering these underlying events and processes. Among those chemicals, the group labeled as endocrine disrupting chemicals (EDCs) offers the clearest evidence of such selectivity, a consequence of their actions on the endogenous sex steroids, androgens and estrogens. Two EDCs in particular offer useful and intriguing examples. One is phthalate esters. The other is bisphenol A. Both agents are used extensively in plastics manufacture, and are pervasive in the environment. Both are produced in immense quantities. Both are found in almost all humans. Phthalates are considered to function in essence as anti-androgens, while bisphenol A is labeled as an estrogen. Their associations with brain sexual differentiation are reviewed and further questions noted. Both EDCs produce a wider spectrum of health effects, however, than would be extrapolated simply from their properties as anti-androgens and estrogens. Obesity is one example. Further complicating their assessment as health risks are questions about nonmonotonic dose-response functions and about transgenerational effects incurred via epigenetic mechanisms. All these facets of endocrine disruption are pieces of a puzzle that challenge neurotoxicologists for solutions.

Sexual dimorphism of oligodendrocytes is mediated by differential regulation of signaling pathways

Sexual dimorphism of white matter has not been considered important, the assumption being that sex hormones are not essential for glial development. We recently showed exogenous hormones in vivo differentially regulate in male and female rodents the life span of oligodendrocytes (Olgs) and amount of myelin (Cerghet et al. [2006] J. Neurosci. 26:1439-1447). To determine which hormones regulate male and female Olg development, we prepared enriched Olg cultures grown in serum-free medium with estrogen (E2), progesterone (P2), and dihydrotestosterone (DHT) or their combinations. P2 significantly increased the number of Olgs in both sexes, but more so in females; E2 had minor effects on Olg numbers; and DHT reduced Olgs numbers in both sexes, but more so in females. Combinations of hormones affected Olg numbers differently from single hormones. The change in Olg numbers was due to changes not in proliferation but rather in survival. P2 increased pAKT by many-fold, but MAPK levels were unchanged, indicating that activation of the Akt pathway by P2 is sufficient to regulate Olg differentiation. DHT reduced pAkt in both sexes but differentially increased pMAPK in males and decreased it in females. Stressing Olgs reveals that both sexes are protected by P2, but females are slightly better protected than males. Females always showed greater differences than males regarding changes in Olg numbers and in signaling molecules. Given the greater fluctuation of neurosteroids in women than in men and the higher incidence of multiple sclerosis (MS) in women, these sexually dimorphic differences may contribute to differences in male and female MS lesions.

Organizational influence of the postnatal testosterone surge on the circadian rhythm of core body temperature of adult male rats

The suprachiasmatic nucleus (SCN) of the hypothalamus coordinates physiological and behavioral circadian rhythms such as activity, body temperature, and hormone secretion. Circadian rhythms coordinated by the SCN often show sex differences arising from both organizational and activational effects of gonadal hormones. In males, little is known about the organizational role of testosterone on the circadian regulation of core body temperature (CBT) in adulthood. To explore this, we castrated or sham-operated male rats on the day of birth, and at 4 months of age, implanted them with transmitters that measured CBT rhythms under a 12:12 light/dark cycle. This study revealed a significantly earlier rise in CBT during the light phase in neonatally castrated males. Subsequently, we found that treating neonatally castrated males with testosterone propionate (TP) in adulthood did not reverse the effect of neonatal castration, thus indicating an organizational role for testosterone. In contrast, a single injection of TP at the time of neonatal surgery, to mimic the postnatal surge of testosterone, coupled with TP treatment in adulthood, normalized the circadian rise in CBT. In a final study we examined CBT circadian rhythms in intact adult male and female rats and detected no differences in the rise of CBT during the light phase, although there was a greater overall elevation in female CBT. Together, results of these studies reveal an early organizational role of testosterone in males on the timing of the circadian rise of CBT, a difference that does not appear to reflect "defeminization".

The aromatase knockout (ArKO) mouse provides new evidence that estrogens are required for the development of the female brain

The classic view of sexual differentiation is that the male brain develops under the influence of testicular secretions, whereas the female brain develops in the absence of any hormonal stimulation. However, several studies have suggested a possible role of estradiol in female neural development, although they did not provide unequivocal evidence that estradiol is indispensable for the development of the female brain and behavior. As a result, the hypothesis subsequently languished because of the lack of a suitable animal model to test estrogen's possible contribution to female differentiation. The recent introduction of the aromatase knockout (ArKO) mouse, which is deficient in aromatase activity because of a targeted mutation in the CYP19 gene and therefore cannot aromatize androgen to estrogen, has provided a new opportunity to reopen the debate of whether estradiol contributes to the development of the female brain. Female ArKO mice showed reduced levels of lordosis behavior after adult treatment with estradiol and progesterone, suggesting that estradiol is required for the development of the neural mechanisms controlling this behavior in female mice. The neural systems affected may include the olfactory systems in that ArKO females also showed impairments in olfactory investigation of odors from conspecifics. Thus, the classic view of sexual differentiation, that is, the female brain develops in the absence of any hormonal secretion, needs to be re-examined.

Sex differences in the incidence of total callosal agenesis in BALB/cCF mice

Corpus callosum (CC) development and adult morphology seems to be affected by sex. Here we analyzed the incidence of total callosal agenesis in 341 adult male and 318 female BALB/cCF mice. This strain of mice presents total or partial callosal agenesis in approximately 20-30% of its population. No significant differences were found in overall distributions of CC lengths and in average callosal lengths (totally acallosal excluded) between male and female mice. However, a highly significant difference in the incidence of total callosal agenesis was demonstrated: 18% (n=56) of the female mice presented such trait as opposed to 10% of males (n=34). This last result suggests that sex is a relevant factor in callosal development in its earliest stages of formation.

Neonatal estrogen blockade prevents normal callosal responsiveness to estradiol in adulthood

The rat corpus callosum (CC) is larger in males than females, and is responsive to hormone manipulations during development. We previously demonstrated that P25 ovariectomy (Ovx) enlarged (defeminized) adult CC, while P70 ovary transfer (OvT) counteracted this enlarging effect, resulting in smaller (feminized) CC. Since OvT females were not Ovx'd until P25, they received some neonatal estrogen (E) exposure. Behavioral data suggest that adult responsiveness to ovarian hormones depends upon prior organization by neonatal E. It has not been determined whether a similar phenomenon occurs for the feminization of brain morphology. The current experiment examined whether our previous finding of adult CC responsiveness to ovarian hormones depended upon neonatal E exposure. We investigated this by assessing the effects of P70 ovarian hormone replacement (via ovary transfer or E pellet) in females that received either (1) normal ovarian hormone exposure until P25 Ovx, or (2) the E receptor blocker tamoxifen from birth to P25 Ovx. Females receiving normal neonatal hormone exposure responded to P70 E in the female-typical manner: E reduced CC size. In contrast, females receiving neonatal E blockade responded to adult E in the opposite manner: E increased CC size. As far as we are aware, this is the first report suggesting that neonatal E exposure organizes the female brain so that it responds normally to the organizing actions of E when later exposure occurs. These findings further challenge the traditional model of female brain development, which asserts that normal female brain organization occurs by default, in the absence of gonadal hormone exposure.

Adult ovary transfer counteracts the callosal enlargement resulting from prepubertal ovariectomy

The rat corpus callosum (CC) is larger in males than females, and is sensitive to hormone manipulations during development. Previous research found that, in rats, CC sensitivity to testosterone ended by postnatal day 8 (P8). In contrast, more recent findings demonstrated that CC responsivity to ovarian hormones continued at least through P70. The current experiment extends these findings by showing that the female callosum is still sensitive to ovarian hormones as late as P130, well into adulthood.

Ovarian hormones can organize the rat corpus callosum in adulthood

The rat corpus callosum (CC) is larger in males than females, and is responsive to hormone manipulations during development. Previous data suggest that CC sensitivity to testosterone ends by postnatal day 8 (P8). In contrast, responsivity to ovarian hormones extends as late as P25. The current series of experiments investigates whether ovarian hormone effects on the callosum are permanent and whether CC sensitivity to ovarian hormones extends beyond P25. We found that P70 ovariectomy (Ovx) did not affect callosal size, suggesting that ovarian hormone exposure sometime prior to P70 is sufficient to feminize the CC, and that once the callosum is feminized, the effects can not be reversed. We also found that P25 ovariectomy enlarged, or defeminized, adult female CC, whereas ovary transfer starting on P55 or P70 counteracted this enlarging effect, resulting in feminized adult CC. Thus, although a previously feminized callosum is not affected by P70 ovarian hormone removal, a not-yet feminized callosum can still be feminized after P70. These findings indicate that there is flexibility in the developmental window within which the female brain is responsive to the active feminization process initiated by ovarian hormones.

Early androgen treatment decreases cognitive function and catecholamine innervation in an animal model of ADHD

The spontaneously hypertensive rat (SHR) has been used as an animal model of attention deficit hyperactivity disorder (ADHD). The present study was designed to determine whether exposure to elevated androgen levels early in development demonstrated impairments in cognitive functioning, neuroendocrine control, and brain development parallel to those seen in ADHD children. The animals (SHR and Wistar (WKY) controls) were implanted with testosterone on postnatal day 10 and tested for behavior in a spatial cognition paradigm on postnatal day 45. Plasma samples were collected for determination of adrenocorticotrophin hormone (ACTH) and corticosterone levels as indicators of the basal tone of the pituitary-adrenal neuroendocrine axis. In addition, the density of tyrosine hydroxylase-immunoreactive fibers (an indicator of catecholamine innervation) in the frontal cortex was compared between animals. The current data show that early testosterone treatment in SHR animals resulted in additional deficits in spatial memory in the water maze, but was ineffective in altering the response of WKY animals. Furthermore, SHR rats had high basal ACTH and low corticosterone levels that may indicate a dysfunctional stress axis similar to other reports in humans with persistent ADHD. Finally, there was a further suppression of tyrosine hydroxylase-immunoreactivity in the frontal cortex of androgen-treated SHR rats. These results support the hypothesis that early androgen treatment may support the neurobiology of animals with genetic predisposition to hyperactivity, impulsivity and inattention in a manner consistent with the enhanced expression of ADHD-like behaviors.

Imprinting by neonatal sex steroids on the structure and function of the mature mouse prostate

Perinatal sex-steroid exposure may result in permanent modifications in the structure and function of the prostate gland. The mechanism of such long-range alterations in hormonal sensitivity is not known. This study aimed to define the molecular requirements for neonatal sex-steroid imprinting and to investigate whether combined administration of neonatal androgens and estrogens had synergistic effects upon the mature mouse prostate. Since the interaction between endogenous and exogenous sex steroids in normal mice makes it difficult to dissociate direct from indirect effects, we used the hypogonadal (hpg) mouse, characterized by congenital androgen deficiency yet still fully responsive to exogenous androgens. Newborn mice (Days 1-2) were administered a single s.c. injection of androgens alone or in combination with an estrogen followed by testosterone-induced maximal prostate growth at maturity. The final effects were determined in 7-wk-old mice through study of ductal architecture in microdissected ventral prostates (VP) and quantitation of volume densities and diameters of prostate tissue components. A single neonatal dose of androgens, but not of estrogen, increased branching morphogenesis and VP weights at adulthood. These effects did not differ significantly between various androgens; in addition, combined androgen and estrogen treatment failed to demonstrate any synergistic effects on the prostate. We conclude that neonatal androgens induce long-range effects upon the mature VP structure as well as its secretory function and that this imprinting occurs via the androgen receptor without requiring aromatization of androgens. However, these conclusions, based on a specific treatment protocol, are confined only to the distal segment of VP, and effects of neonatal sex-steroid exposure in other regions or lobes of VP may differ.

Sex and species differences in mouse and rat forebrain commissures depend on the method of adjusting for brain size

Sex differences in the forebrain commissures (corpus callosum, hippocampal commissure, and anterior commissure) were examined in B6D2F2 hybrid mice and Sprague-Dawley rats. Twenty-four male-female littermate pairs of mice were perfused at each of 21, 42 and 63 days of age and the midsagittal area of the commissures was measured from en bloc stained tissue. Twenty-two male-female littermate pairs of rats were examined at 110 days of age using the same methods. Male mice had larger bodies than females but no sex differences were found for mouse brain weight or commissure areas. In contrast, a significant sex difference was found for rat body, brain, corpus callosum and hippocampal commissure sizes. Four methods were used to adjust for differences in brain size (ratio, geometric, linear regression, and allometric). When the two species were analysed separately, neither mice nor rats showed significant sex differences in commissure areas relative to brain size if regression or allometric adjustments were made. Even when data from mice and rats were combined into one large group with a wide range of values, no species or sex differences were apparent after adjustments were made for brain size with either the regression or allometric methods. The use of ratios to adjust for differences in overall size is not recommended, especially because this method does not effectively remove the influence of brain size from commissure size; a substantial correlation is often present between the ratio and brain size.

The size of the splenium of the rat corpus callosum: influence of hormones, sex ratio, and neonatal cryoanesthesia

The splenium (posterior 1/5) of the corpus callosum is sexually dimorphic in the adult rat brain. In the present study we examined the role of developmental hormones and cryoanesthesia (which is normally used during the performance of neonatal hormone manipulations) on the gross size of the splenium in male and female rats. There was a sex difference in splenial size (male > female) among nonhormonally manipulated animals, p = .0007. While neonatal castration was ineffective in altering the size of the male splenium, testosterone injections in females were found to increase the size of the splenium relative to oil-injected females, p = .05. The effect of developmental testosterone was further observed: Sex ratio (males to females) of the litter correlated with splenial area in females, r = .55, p = .03. Duration of cryoanesthesia negatively correlated with splenial area in males, r = -.81, p = .03, with a similar trend in females.

Glucocorticoid receptor mRNA and protein in fetal rat lung in vivo: modulation by glucocorticoid and androgen

Pulmonary glucocorticoid receptor (GR) is essential to timely preparation for the onset of breathing air at birth. We have previously used primary culture of late-gestation fetal rat lung cells to demonstrate differential regulation of GR by glucocorticoid depending on cell type. In this study, we hypothesized that the action of glucocorticoid on GR mRNA expression and protein elaboration in lung cells might be modulated by interactions present in vivo but not in primary culture. Given that male sex hormone (androgen) has an inhibitory effect on antenatal lung development, we also postulated that androgen would decrease antenatal lung GR. We report that antenatal maternal injection of the glucocorticoid dexamethasone (1 mg/kg) enhanced fetal lung cellular levels of GR mRNA and protein as assessed by in situ hybridization and immunocytochemistry (ICC), respectively. ICC was performed using polyclonal rabbit anti-human antibody that reacts with rat GR whether bound to ligand or not and does not interfere with GR binding to DNA. Levels of GR mRNA and protein were enhanced in cells throughout all areas of the lung tissue, suggesting that interactions occurring in intact tissue may override the previously reported direct inhibition by glucocorticoid of GR protein elaboration in isolated fetal rat lung epithelial cells. Furthermore, antenatal administration of the androgen 5alpha-dihydrotestosterone (0.2 mg/kg) reduced tissue levels of GR mRNA and protein, consistent with androgenic inhibition of antenatal lung development by decreasing GR. We conclude that glucocorticoids and androgens exert opposite effects on fetal lung GR.