Plasma testosterone in fetal rats and their mothers on day 19 of gestation

Phosphate, Calcium, and Vitamin D: Key Regulators of Fetal and Placental Development in Mammals

Normal calcium and bone homeostasis in the adult is virtually fully explained by the interactions of several key regulatory hormones, including parathyroid hormone, 1,25 dihydroxy vitamin D3, fibroblast growth factor-23, calcitonin, and sex steroids (estradiol and testosterone). In utero, bone and mineral metabolism is regulated differently from the adult. During development, it is the placenta and not the fetal kidneys, intestines, or skeleton that is the primary source of minerals for the fetus. The placenta is able to meet the almost inexhaustible needs of the fetus for minerals by actively driving the transport of calcium and phosphorus from the maternal circulation to the growing fetus. These fundamentally important minerals are maintained in the fetal circulation at higher concentrations than those in maternal blood. Maintenance of these inordinately higher fetal levels is necessary for the developing skeleton to accrue sufficient minerals by term. Importantly, in livestock species, prenatal mineralization of the skeleton is crucial for the high levels of offspring activity soon after birth. Calcium is required for mineralization, as well as a plethora of other physiological functions. Placental calcium and phosphate transport are regulated by several mechanisms that are discussed in this review. It is clear that phosphate and calcium metabolism is intimately interrelated and, therefore, placental transport of these minerals cannot be considered in isolation.

Non-invasive Fecal Steroid Measurements for Monitoring the Reproductive Status of a Critically Endangered Yangtze Finless Porpoises (Neophocaena asiaeorientalis asiaeorientalis)

Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis) is a critically endangered freshwater cetacean dwelling in the Yangtze River and its adjoining lakes. Affected mainly by the various anthropogenic activities in this region, its population decreased dramatically in the past two decades. To protect this animal from extinction, captive breeding program is an important way to provide basic knowledge for wild population conservation. Non-invasive fecal steroid radioimmunoassay technique was validated in three captive Yangtze finless porpoises for the first time in this study. The seasonality of one captive male and the reproductive status of two females were investigated by longitudinal monitoring their fecal reproductive steroid hormone metabolites. Pregnancy could be diagnosed by an abrupt increase in fecal progesterone metabolites. In late pregnancy (4 months before birth), a significant decrease of fecal progesterone metabolites was observed, which might be referenced for the expectation of parturition date. Seven estrous cycles were recognized in one breeding season of a non-pregnant female judged by the variation of fecal progesterone metabolite levels. The fecal progesterone metabolite level was proved a reliable and precise indicator for estrus and pregnancy diagnosis.

Non-model species deliver a non-model result: Nutria female fetuses neighboring males in utero have lower testosterone

Neighboring fetuses may impact their siblings in various respects, depending on their in utero location and sex. The effects of the intrauterine position (IUP) are widely studied in model organisms, especially laboratory bred murine strains that are characterized by short gestations and altricial offspring. In some species, the proximity to a male fetus and its higher circulating testosterone masculinizes neighboring female fetuses. In utero testosterone exposure might be manifested as higher testosterone concentrations, which contribute to a variation in morphology, reproductive potential and behavior. In this study, we examined the influence of neighboring an opposite sex fetus on testosterone levels in a feral animal model characterized by a long gestation and precocious offspring. Using necropsies of culled nutria (Myocastor coypus), we accurately determined the IUP and quantified testosterone immunoreactivity in fetal hair. We found that as expected, both male and female fetuses neighboring a male in utero had longer anogenital distance. However, females adjacent to males in utero showed lower testosterone levels than male fetuses, while testosterone levels of females without a male neighbor did not differ from those of males. This surprising result suggests an alternative mode by which local exogenous steroids may modify the local fetal environment. Our study emphasizes the importance of examining known phenomena in species with different life histories, other than the traditional murine models, to enhance our understanding of the evolutionary mechanisms that are driving sexual differentiation.

Evidence that prenatal testosterone transfer from male twins reduces the fertility and socioeconomic success of their female co-twins

During sensitive periods in utero, gonadal steroids help organize biological sex differences in humans and other mammals. In litter-bearing species, chromosomal females passively exposed to prenatal testosterone from male littermates exhibit altered physical and behavioral traits as adults. The consequences of such effects are less well understood in humans, but recent near-doubling of twinning rates in many countries since 1980, secondary to advanced maternal age and increased reliance on in vitro fertilization, means that an increasing subset of females in many populations may be exposed to prenatal testosterone from their male co-twin. Here we use data on all births in Norway (n = 728,842, including 13,800 twins) between 1967 and 1978 to show that females exposed in utero to a male co-twin have a decreased probability of graduating from high school (15.2%), completing college (3.9%), and being married (11.7%), and have lower fertility (5.8%) and life-cycle earnings (8.6%). These relationships remain unchanged among the subsets of 583 and 239 females whose male co-twin died during the first postnatal year and first 28 days of life, respectively, supporting the interpretation that they are due primarily to prenatal exposure rather than to postnatal socialization effects of being raised with a male sibling. Our findings provide empirical evidence, using objectively measured nation-level data, that human females exposed prenatally to a male co-twin experience long-term changes in marriage, fertility, and human capital. These findings support the hypothesis of in utero testosterone transfer between twins, which is likely affecting a small but growing subset of females worldwide.

Influence of Low Protein Diet-Induced Fetal Growth Restriction on the Neuroplacental Corticosterone Axis in the Rat

Objectives: Placental steroid metabolism is linked to the fetal hypothalamus-pituitary-adrenal axis. Intrauterine growth restriction (IUGR) might alter this cross-talk and lead to maternal stress, in turn contributing to the pathogenesis of anxiety-related disorders of the offspring, which might be mediated by fetal overexposure to, or a reduced local enzymatic protection against maternal glucocorticoids. So far, direct evidence of altered levels of circulating/local glucocorticoids is scarce. Liquid chromatography tandem-mass spectrometry (LC-MS/MS) allows quantitative endocrine assessment of blood and tissue. Using a rat model of maternal protein restriction (low protein [LP] vs. normal protein [NP]) to induce IUGR, we analyzed fetal and maternal steroid levels via LC-MS/MS along with the local expression of 11beta-hydroxysteroid-dehydrogenase (Hsd11b). Methods: Pregnant Wistar dams were fed a low protein (8%, LP; IUGR) or an isocaloric normal protein diet (17%, NP; controls). At E18.5, the expression of Hsd11b1 and 2 was determined by RT-PCR in fetal placenta and brain. Steroid profiling of maternal and fetal whole blood, fetal brain, and placenta was performed via LC-MS/MS. Results: In animals with LP-induced reduced body (p < 0.001) and placental weights (p < 0.05) we did not observe any difference in the expressional Hsd11b1/2-ratio in brain or placenta. Moreover, LP diet did not alter corticosterone (Cort) or 11-dehydrocorticosterone (DH-Cort) levels in dams, while fetal whole blood levels of Cort were significantly lower in the LP group (p < 0.001) and concomitantly in LP brain (p = 0.003) and LP placenta (p = 0.002). Maternal and fetal progesterone levels (whole blood and tissue) were not influenced by LP diet. Conclusion: Various rat models of intrauterine stress show profound alterations in placental Hsd11b2 gatekeeper function and fetal overexposure to corticosterone. In contrast, LP diet in our model induced IUGR without altering maternal steroid levels or placental enzymatic glucocorticoid barrier function. In fact, IUGR offspring showed significantly reduced levels of circulating and local corticosterone. Thus, our LP model might not represent a genuine model of intrauterine stress. Hypothetically, the observed changes might reflect a fetal attempt to maintain anabolic conditions in the light of protein restriction to sustain regular brain development. This may contribute to fetal origins of later neurodevelopmental sequelae.

Prenatal androgen-receptor activity has organizational morphological effects in mice

Prenatal sex hormones exert organizational effects. It has been suggested that prenatal sex hormones affect adult morphological parameters, such as the finger length. Especially the second-to-fourth finger length (2D:4D) ratio has been implicated to be modified when exposed to higher androgen levels in utero. Here we show in a mouse model that experimental manipulation of the prenatal androgen level, by blocking the androgen receptor with flutamide or activating the androgen receptor with dihydrotestosterone (DHT), leads to changes in the length of the fingers of all paws in males and females. In addition to that, also total paw length and the 2D:4D ratio was affected. In males treated with DHT, the 2D:4D ratio was increased, while flutamide-treatment in females led to a reduced 2D:4D ratio. We also measured other parameters, such as head size, body length and tail length and demonstrate that body morphology is affected by prenatal androgen exposure with more prominent effects in females. Another factor that is thought to be influenced by early androgens is handedness. We tested mice for handedness, but did not find a significant effect of the prenatal treatment. These findings demonstrate that prenatal androgen activity is involved in the development of body morphology and might be a useful marker for prenatal androgen exposure.

Characterization of estrogens, testosterone, and cortisol in normal bottlenose dolphin (Tursiops truncatus) pregnancy

The goal of this study was to describe profiles of serum estrogens, testosterone and cortisol during normal pregnancy in bottlenose dolphins. Predominant estrogens in all categories of dolphin sera pools during estrus and pregnancy (EARLY: Days 0-120; MID: Days 121-240; LATE: Days 241 to parturition; Day 0=day of conception) were estrone/estrone conjugates (E1-C) and estriol (E3). Serum samples collected throughout 101 normal pregnancies were analyzed for E1-C, E3, testosterone (T) and cortisol (CORT). E1-C was higher (P<0.05) during LATE compared to EARLY and MID, and higher (P<0.05) in nulliparous than multiparous females. E1-C concentrations were also inversely associated with maternal age (P=0.05). E3 was higher (P<0.05) in EARLY than MID and LATE, and higher overall for nulliparous than multiparous females, but concentrations were similar among gestational stages when parity was excluded from analyses. Analysis by indexed month post-conception (IMPC) demonstrated that E1-C increased from IMPC 9 and peaked at IMPC 11. E3 was significantly elevated during IMPC 1, decreased until IMPC 6 and peaked at IMPC 11. T increased (P<0.05) at IMPC 3 and continued to increase throughout gestation (P<0.05). CORT was higher (P<0.05) during LATE compared to EARLY and MID (P<0.05), peaked during IMPC 12, and was not affected by parity. Hormone profiles were not influenced by fetal sex.

Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones

Mineral and bone metabolism are regulated differently in utero compared with the adult. The fetal kidneys, intestines, and skeleton are not dominant sources of mineral supply for the fetus. Instead, the placenta meets the fetal need for mineral by actively transporting calcium, phosphorus, and magnesium from the maternal circulation. These minerals are maintained in the fetal circulation at higher concentrations than in the mother and normal adult, and such high levels appear necessary for the developing skeleton to accrete a normal amount of mineral by term. Parathyroid hormone (PTH) and calcitriol circulate at low concentrations in the fetal circulation. Fetal bone development and the regulation of serum minerals are critically dependent on PTH and PTH-related protein, but not vitamin D/calcitriol, fibroblast growth factor-23, calcitonin, or the sex steroids. After birth, the serum calcium falls and phosphorus rises before gradually reaching adult values over the subsequent 24-48 h. The intestines are the main source of mineral for the neonate, while the kidneys reabsorb mineral, and bone turnover contributes mineral to the circulation. This switch in the regulation of mineral homeostasis is triggered by loss of the placenta and a postnatal fall in serum calcium, and is followed in sequence by a rise in PTH and then an increase in calcitriol. Intestinal calcium absorption is initially a passive process facilitated by lactose, but later becomes active and calcitriol-dependent. However, calcitriol's role can be bypassed by increasing the calcium content of the diet, or by parenteral administration of calcium.

Maternal gestational cortisol and testosterone are associated with trade-offs in offspring sex and number in a free-living rodent (Urocitellus richardsonii)

The adaptive manipulation of offspring sex and number has been of considerable interest to ecologists and evolutionary biologists. The physiological mechanisms that translate maternal condition and environmental cues into adaptive responses in offspring sex and number, however, remain obscure. In mammals, research into the mechanisms responsible for adaptive sex allocation has focused on two major endocrine axes: the hypothalamic pituitary adrenal (HPA) axis and glucocorticoids, and the hypothalamic pituitary gonadal (HPG) axis and sex steroids, particularly testosterone. While stress-induced activation of the HPA axis provides an intuitive model for sex ratio and litter size adjustment, plasma glucocorticoids exist in both bound and free fractions, and may be acting indirectly, for example by affecting plasma glucose levels. Furthermore, in female mammals, activation of the HPA axis stimulates the secretion of adrenal testosterone in addition to glucocorticoids (GCs). To begin to untangle these physiological mechanisms influencing offspring sex and number, we simultaneously examined fecal glucocorticoid metabolites, free and bound plasma cortisol, free testosterone, and plasma glucose concentration during both gestation and lactation in a free-living rodent (Urocitellus richardsonii). We also collected data on offspring sex and litter size from focal females and from a larger study population. Consistent with previous work in this population, we found evidence for a trade-off between offspring sex and number, as well as positive and negative correlations between glucocorticoids and sex ratio and litter size, respectively, during gestation (but not lactation). We also observed a negative relationship between testosterone and litter size during gestation (but not lactation), but no effect of glucose on either sex ratio or litter size. Our findings highlight the importance of binding proteins, cross-talk between endocrine systems, and temporal windows in the regulation of trade-offs in offspring sex and number.

Maternal gestational androgen levels in female marmosets (Callithrix geoffroyi) vary across trimesters but do not vary with the sex ratio of litters

Maternal hormones can dramatically modify offspring phenotypes via organizational actions on morphological and behavioral development. In placental mammals, there is the possibility that some portion of hormones in maternal circulation may be derived from fetal origin. We tested the possibility that maternal androgens in pregnant female marmosets reflected, in part, contributions from male fetuses by comparing levels of urinary androgens across pregnancy in females carrying varying numbers of male offspring. We monitored urinary androgen excretion in 18 pregnancies from five female white-faced marmosets (Callithrix geoffroyi). Androgen levels rose significantly in the first trimester of pregnancy, reached a peak in the middle of the second trimester, and then declined gradually until parturition. At no point in pregnancy were levels of urinary androgens higher in females carrying litters that had 50% or more males than in females carrying litters that were less than 50% male. Levels of maternal androgens were not associated with litter size, the number of males in the litter, or with the proportion of the litter that was male. The high levels of androgen in pregnant females are therefore likely of strictly maternal origin, and any modification of fetal growth and development can be considered a 'maternal effect'.

Maternal corticosterone but not testosterone level is associated with the ratio of second-to-fourth digit length (2D:4D) in field vole offspring (Microtus agrestis)

The steroid environment encountered by a foetus can strongly affect its post-natal physiology and behaviour. It has been proposed that steroid concentrations experienced in utero could be estimated from adults by measuring their second-to-fourth digit length ratio (2D:4D). However, there is still little direct evidence that intra-uterine steroid levels affect individual 2D:4D. We examined whether maternal pre-pregnancy testosterone and corticosterone levels (as estimates of intra-uterine testosterone and corticosterone exposure) affected the 2D:4D of pups in non-domesticated field voles (Microtus agrestis), measured by X-rays at the age of weaning (21 days). Furthermore, for the first time in a non-human species, we studied whether testosterone and corticosterone levels correlated with 2D:4D in adult females. We found that the maternal pre-pregnancy level of testosterone was not associated with offspring 2D:4D in either the left or the right paw. Instead, maternal pre-pregnancy corticosterone level was positively correlated with offspring 2D:4D in the right paw, but unrelated to 2D:4D in the left paw. In addition, the 2D:4D of adult females was not associated with either their circulating testosterone or corticosterone levels. Our results suggest that in field voles maternally administered testosterone is not a major determinant of offspring 2D:4D, whereas maternal stress appears to account for some of the variation in the 2D:4D of their offspring.

Androgen receptor and 17beta-HSD type 2 regulation in neonatal mouse lung development

A QPCR analysis of androgen receptor and several androgen metabolizing genes was performed during the saccular and alveolar stages of mouse lung development. Androgen receptor expression showed a statistically significant increase during the alveolar stage while levels of 17beta-hydroxysteroid dehydrogenase type 2 (17beta-HSD 2) expression significantly decreased at the end of the saccular stage and remained low throughout the alveolar period. 17beta-HSD 1, 17beta-HSD 5, 5alpha-reductase type 1, and mouse 3alpha-HSD did not present such a regulation. The androgen receptor protein was primarily detected in the nucleus of airway epithelial cells and of a subset of respiratory epithelial cells. 17beta-HSD 2 mRNA co-localized with androgen receptor protein during saccularization, but was absent from airway epithelium during alveolarization. Taken together, our results demonstrate temporal and spatial regulation of androgen receptor and 17beta-HSD 2 during the sacculo-alveolar transition period of mouse lung development suggesting control of androgen action.

Endogenous oestradiol regulates progesterone receptor expression in the brain of female rat fetuses: what is the source of oestradiol?

Testosterone secreted by male testes during fetal development is aromatized to oestradiol (E(2)) or reduced to the androgen, dihydrotestosteorne (DHT), within specific tissues. The female brain is assumed to develop in the relative absence of gonadal steroid hormones, as the ovary is steroidogenically quiescent until later in postnatal life. However, the proximity of a female fetus to male littermates in utero can increase her exposure to testosterone, and thereby its metabolites. To date, it is has been difficult to dissociate the effects of male-derived E(2) from those of DHT on the developing female brain. In the present study, anogential distance (AGD) in females was used as an androgen-dependent bioassay, whereas progesterone receptor (PR) expression within the medial preoptic nucleus (MPN) was used as an E-dependent measure. Pregnant dams received the aromatase inhibitor, 1,4,6-androstatriene-3,17-dione (ATD), or vehicle from embryonic day 16 (ED16) to ED21. On ED22, AGD and PR-immunoreactivity (-ir) were measured in females that had zero, one, or two males (0-2M) or females that had three, four, or five males (3-5M) in the uterine horn. AGD was significantly greater in 3-5M females compared to 0-2M females, suggesting that male littermates are the source of androgenic exposure in the female fetus. ATD treatment significantly decreased PR-ir in the MPN, demonstrating E(2) regulation of PR. However, the total number of males in the uterine horn did not effect PR expression. There was no correlation between PR-ir and AGD, suggesting that these measures are influenced independently. Together, these results suggest that although male littermates provide a significant source of androgens to female fetuses, the amount of E(2) aromatized from male-derived testosterone may not be the only biologically relevant source of androgens or E(2). Alternative sources of E(2) may be essential in ensuring the normal development of the female brain.

Dihydrotestosterone Stimulates Proliferation and Differentiation of Fetal Calvarial Osteoblasts and Dural Cells and Induces Cranial Suture Fusion

BACKGROUND

The higher prevalence of metopic and sagittal suture synostosis in male infants suggests a role for androgens in early craniofacial development. These experiments characterize the influence of androgen stimulation on growth and differentiation of fetal dural and calvarial bone cells and on cranial suture fusion.

METHODS

Primary murine fetal (E18) dural cells and calvarial osteoblasts were isolated and cultured. Cells were treated for 48 hours with 5alpha-dihydrotestosterone (0 to 1000 nM). Cell proliferation was examined by nonradioactive proliferation assay; mRNA expression of alkaline phosphatase, transforming growth factor (TGF)-beta1, and the bone matrix proteins osteopontin, osteocalcin, and type 1 collagen was determined by reverse-transcriptase polymerase chain reaction. In separate experiments, intact fetal calvariae were grown in tissue culture with 10 nM 5alpha-dihydrotestosterone for 7 and 14 days and then examined histologically.

RESULTS

Androgen stimulation at 5 nM increased proliferation of fetal dural cells by 46.0 percent and of fetal calvarial osteoblasts by 20.5 percent. Dural expression of osteopontin, osteocalcin, and type 1 collagen was enhanced by 5alpha-dihydrotestosterone, as was that of TGF-beta1 and alkaline phosphatase. Androgen stimulation increased calvarial osteoblast expression of alkaline phosphatase and TGF-beta1 but induced little change in expression of osteocalcin, osteopontin, and type 1 collagen. In tissue culture, 5alpha-dihydrotestosterone stimulated osteoid formation and fusion of sagittal sutures.

CONCLUSIONS

Androgen stimulation of dural cells and osteoblasts isolated from fetal calvaria promotes cell proliferation and osteoblastic differentiation and can induce cranial suture fusion. These results suggest that sex steroid hormone signaling may stimulate sutural osteogenesis by means of osteodifferentiation of dural cells, thus explaining the male prevalence of nonsyndromic craniosynostosis.

Effect of intrauterine position on sex differences in the gabaergic system and behavior of rats

In multiparous species such as the rat (in this case the albino Wistar strain), steroid influence during fetal growth is affected by the relative intrauterine position of male and female fetuses and is stronger when the potential effects of contiguity and caudal position are combined. The effect of intrauterine position on gonadal steroid levels in neonatal and adult animals was examined using radioimmunoassay techniques. Since the organizing effect of prenatal steroids may influence the postnatal GABA content, HPLC was used to determine the gabaergic content in several hypothalamic and limbic areas in the adult rat. The effects of intrauterine position on adaptive behavior were examined by recording exploratory behavior (using the corridor and hole board tests) and intraspecific aggression (induced by isolation). Female pups influenced by males during development produced more testosterone. In adult males, those that developed closer to the cervix (and with no influence from other fetuses) produced more testosterone and less estradiol. The same animals also produced more hypothalamic GABA and showed greater exploratory capacity. No significant differences were seen between any experimental groups with respect to aggression. These results show increased variability between males with respect to adult exploratory behavior, and in the neurochemical and endocrine systems involved, due to intrauterine position during development. The effect of this physiological phenomenon on the structure of rodent populations is discussed.

Intrauterine position and postnatal growth in Sprague–Dawley rats and ICR mice

In rodents, steroid hormones are thought to be transported between adjacent fetuses, and male or female fetuses that develop in utero between female fetuses may have higher serum levels of estradiol, and lower serum levels of testosterone, relative to siblings of the same sex that develop between two male fetuses. The consequence in the variation of postnatal growth, development, and function in the intrauterine position, using various parameters such as anogenital distance, preputial separation and vaginal opening, estrous cycle, locomotor activity, and growth of reproductive organs, were examined in Sprague-Dawley rats. ICR mice were treated with 17beta-estradiol before copulation and during pregnancy to address the interaction with endogenous estradiol during pregnancy. In rats, no evidence of effects of prior intrauterine position was observed for any of the parameters examined. Mouse fetal exposure via the mother to low-dose 17beta-estradiol revealed no changes in the rate of postnatal growth in males and females that developed in any intrauterine position in utero. The results of this study suggested that the intrauterine position of the embryos/fetuses did not affect the postnatal growth of the reproductive organs, sexual maturation, or behavior in rats and mice.

The number of male pups within a litter of NMRI mice is associated with the dam's food preferences late in pregnancy

Pregnancy related differences in food preferences and food intake are well recognized in animals and humans. Changes in maternal hormonal profiles are supposed to be implicated in such a phenomenon. Since fetal sex hormones are excreted in their dams' bloodstream, they can affect the behaviour of mothers during gestation. We investigated whether there was a relationship between the number of male pups within the litter and food preferences in pregnant mice. In the first experiment we found that, parcelling out the effect of litter size, the number of male fetuses was positively correlated to maternal testosterone levels on pregnancy day (PD)18. In the second experiment we showed an association between the number of males and the consumption of a 0.25% saccharin solution on PD18, controlling for litter size and baseline (before pregnancy) saccharin intake. In the third experiment, mice on PD18 were placed in a test cage and underwent a 1-h binary choice test between two unknown unpalatable diets, a high fat diet (HFD) and a sweet mesh diet (HSD). Results revealed that the number of males was negatively associated with the preference for the HFD at birth. These results are consistent with the hypothesis that the behavioural profile of dams can be associated with fetal gender composition.

Intrauterine position effects

A review of the literature suggests that individual variability in sex-related traits may be influenced by variations in hormonal exposure during fetal development. In litter-bearing mammals, fetuses develop in utero and may be subjected to differing hormonal environments based upon the sex of neighboring fetuses. Female fetuses developing between two males tend to show masculinized anatomical, physiological and behavioral traits as adults. Female fetuses developing without adjacent males, on the other hand, tend to show more feminized traits as adults. These traits include permanently altered hormone levels, reproductive organs, aggressive behaviors, secondary sex ratios and susceptibility to endocrine disruption. This intrauterine effect is due to the transfer of testosterone from male fetuses to adjacent fetuses. While these effects have been most clearly demonstrated in mice, other rodents and swine also show intrauterine position (IUP) effects. Some of these effects are similar to the influence of prenatal stress on adult phenotypes. A few reports on human twins suggest that variability in some masculine and feminine traits may be due to intrauterine hormonal signals. IUP effects may impact a number of scientific fields of research such as endocrine disruption, toxicology, population biology, animal production and health.

Testosterone and estrogen affect neuronal differentiation but not proliferation in early embryonic cortex of the rat: the possible roles of androgen and estrogen receptors

We examined the effect of testosterone (T) and 17 beta-estradiol (E) on differentiation and proliferation of cultured neurons from the cortex of 14-day-rat embryos (E14) using immunocytochemistry. We found that the cultures receiving E had significantly more neurons with longer neurites than the control cultures, while both fewer and less differentiated neurons were seen after 24 h of incubation with T. However, neither T nor E changed the number of cells positive for BrdU, a proliferation marker. We also found that the androgen receptor (AR) was markedly expressed in the neurons, whereas the expression of estrogen (ER(alpha)) receptor was barely detectable. These results suggest that E and T differ in effect on differentiation, while neither affect proliferation in early developmental cortex. Furthermore, since the AR is expressed in the cortical neurons by E14, the inhibitory effect of T on differentiation may be receptor-mediated, while the stimulatory effects of estrogen in the cortex do not appear to involve nuclear ER(alpha) at this developmental stage.

The expression of GABA(A) receptor alpha2 subunit is upregulated by testosterone in rat cerebral cortex

The GABAergic system is sexually dimorphic in certain brain regions and can be regulated by testosterone (T). However, the contribution of T to sex-specific developmental processes in the brain is less clear. We have examined whether T regulates expression of GABA(A) receptor alpha2 subunit in the cerebral cortex of embryonic and postnatal female rats using in situ hybridization and Western blotting. We found that both alpha2 mRNA and protein levels are significantly increased by T treatment at embryonic day 20 (E20) and birth (P0). The observed modulation of the expression of GABA(A) receptor alpha2 subunit by T may be translated into changes in the levels or composition of GABA(A) receptor, either of which would be expected to alter neuronal functional response to GABA activation. As the effects of T are developmental-stage-specific, they may have an organizational impact on brain development.

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.

5-HT1A receptor mRNA expressions differ in the embryonic spinal cord of male and female rats

During critical periods of development, the effects of testosterone (T) on promoting androgenization of the central nervous system (CNS) are reflected not only by behavior, morphology, and hormone secretion but also by gene expression. The mechanisms involved in sexual differentiation of the CNS, however, remain incompletely defined. The current set of experiments examined with in situ hybridization the dimorphism in 5-HT1A receptor mRNA expression in the embryonic rat spinal cord and the possible role of T in the dimorphism. We found sex-related differences in expression of 5-HT1A mRNA in the spinal cord, which were altered by a single injection of T. The results suggest that this gonadal steroid is responsible for the sexual dimorphism in 5-HT1A mRNA expression occurring during the critical period.

Using a Regression Approach to Study the Influence of Male Fetuses on the Genital Morphology of Neonatal Female Rats

Among newborn female rats considerable variability is found in genital morphology (e.g., anogenital distance, AGD). Presumably, such differences are related to prenatal androgen exposure, with greater exposure resulting in larger AGD's and thus in a trend toward masculinization. The source of prenatal androgen in female fetuses is unclear, but a role for male uterine mates has been implicated. The present study investigated the effect of a number of prenatal factors related to number and position of males in utero on female AGD in two strains of rats. Because such prenatal factors often show systematic covariance, a methodology was used that enabled statistical control over variables that could not be /experimentally controlled. Results confirmed the importance of caudal males to Female AGD,and identified two additional intra-uterine variables salient to female genital masculinization, namely the distance of the female fetus from the nearest caudal male, and the overall number of males sharing the same uterine horn. An increase in number of adjacent males was, contrary to previous reports, associated with a decrease in AGD, but this effect was limited to one strain. There was considerable variation in AGD across the two strains, and, more importantly, across litters, suggesting the importance of factors impacting the litter as a whole rather than specific individuals within the litter.