Influence of fetal position on neonatal androgen-induced sterility and sexual behavior in female rats

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.

The Male Fetal Biomarker INSL3 Reveals Substantial Hormone Exchange between Fetuses in Early Pig Gestation

The peptide hormone INSL3 is uniquely produced by the fetal testis to promote the transabdominal phase of testicular descent. Because it is fetal sex specific, and is present in only very low amounts in the maternal circulation, INSL3 acts as an ideal biomarker with which to monitor the movement of fetal hormones within the pregnant uterus of a polytocous species, the pig. INSL3 production by the fetal testis begins at around GD30. At GD45 of the ca. 114 day gestation, a time at which testicular descent is promoted, INSL3 evidently moves from male to female allantoic compartments, presumably impacting also on the female fetal circulation. At later time-points (GD63, GD92) there is less inter-fetal transfer, although there still appears to be significant INSL3, presumably of male origin, in the plasma of female fetuses. This study thus provides evidence for substantial transfer of a peptide hormone between fetuses, and probably also across the placenta, emphasizing the vulnerability of the fetus to extrinsic hormonal influences within the uterus.

Anogenital distance and condition as predictors of litter sex ratio in two mouse species: a study of the house mouse (Mus musculus) and mound-building mouse (Mus spicilegus)

The Trivers - Willard hypothesis (1973) suggests that the maternal condition may affect the female's litter size and sex ratio. Since then other factors had been found. Previous findings revealed in the case of some mammalian species, that females with larger anogenital distance have smaller litters, while the sex ratio is male-biased. That has only been demonstrated in laboratory animals, while the genetic diversity of a wild population could mask the phenomenon seen in laboratory colonies. We examined the connection between morphological traits (weight and anogenital distance) and the reproductive capacity of two wild mice species, the house mouse and the mound-building mice. We showed in both species that anogenital distance and body weight correlated positively in pre-pubertal females, but not in adults. Neither the house mouse nor the mound-building mouse mothers' weight had effect on their litter's size and sex ratio. Otherwise connection was found between the mothers' anogenital distance and their litters' sex ratio in both species. The results revealed that females with larger anogenital distance delivered male biased litter in both species. The bias occurred as while the number of female pups remained the same; mothers with large anogenital distance delivered more male pups compared to the mothers with small anogenital distance. We concluded that a female's prenatal life affects her reproductive success more than previously anticipated.

Cooperation of sex chromosomal genes and endocrine influences for hypothalamic sexual differentiation

There is little debate that mammalian sexual differentiation starts from the perspective of two primary sexes that correspond to differential sex chromosomes (X versus Y) that lead to individuals with sex typical characteristics. Sex steroid hormones account for most aspects of brain sexual differentiation, however, a growing literature has raised important questions about the role of sex chromosomal genes separate from sex steroid actions. Several important model animals are being used to address these issues and, in particular, they are taking advantage of molecular genetic approaches using different mouse strains. The current review examines the cooperation of genetic and endocrine influences from the perspective of behavioral and morphological hypothalamic sexual differentiation, first in adults and then in development. In the final analysis, there is an ongoing need to account for the influence of hormones in the context of underlying genetic circumstances and null hormone conditions.

Of mice and rats: key species variations in the sexual differentiation of brain and behavior

Mice and rats are important mammalian models in biomedical research. In contrast to other biomedical fields, work on sexual differentiation of brain and behavior has traditionally utilized comparative animal models. As mice are gaining in popularity, it is essential to acknowledge the differences between these two rodents. Here we review neural and behavioral sexual dimorphisms in rats and mice, which highlight species differences and experimental gaps in the literature, that are needed for direct species comparisons. Moving forward, investigators must answer fundamental questions about their chosen organism, and attend to both species and strain differences as they select the optimal animal models for their research questions.

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.

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.

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.

Variation in reproductive traits is associated with short anogenital distance in female rats

Anogenital distance was used as a biomarker for natural variation in prenatal androgenization of female rats and was associated with individual differences in a suite of reproductive characteristics. Females with short anogenital distances were likely to have first vaginal estrus earlier than females with longer anogenital distances and to have first vaginal estrus on the same day as vaginal opening. In young adulthood, females with short anogenital distances had shorter ovarian cycles and less intense lordosis reflexes in response to manual palpation than those with longer anogenital distances, but only when living individually, not in groups. Taken together, these findings indicate that prenatal androgenization affects reproductive traits throughout the life span.

Effects of intrauterine position on the metabolic capacity of the hypothalamus of female gerbils

The intrauterine position that a rodent fetus occupies relative to members of the same or opposite gender affects both its reproductive physiology and behavior when adult. Cytochrome oxidase histochemistry was used to assess regional differences in the oxidative metabolic capacity of the hypothalamus of female Mongolian gerbils that developed in utero between 2 female fetuses (n = 15) or between 2 male fetuses (n = 14). Cytochrome oxidase reactivity was measured densitometrically by experimenters unaware of subject intrauterine position. Gray-to-white matter ratios of optical density in 11 brain regions were used as a normalized index of metabolic capacity. Significant group differences in the metabolic capacity of the medial and the posterior parts of the anterior hypothalamus were revealed. Females that developed in utero between 2 male fetuses showed significant increases (19-22%) in cytochrome oxidase reactivity in these brain regions compared to that in females that developed between 2 female fetuses. The medial part of the anterior hypothalamus contributes to copulatory behavior, whereas the posterior part of the anterior hypothalamus may be involved in the control of pituitary gonadotropin secretion. Both these functions are influenced by intrauterine position during fetal life. To our knowledge, this is the first demonstration of metabolic changes in hypothalamic areas of the adult related to the differences in intrauterine position.

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.

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

Plasma testosterone levels were higher in pooled samples from male fetuses than from female fetuses on day 19 of pregnancy. Plasma testosterone from female fetuses with males located caudally in the uterus was higher than from females that lacked such males. Testosterone level of both male and female fetuses was correlated with maternal testosterone. No correlation was found between maternal testosterone and number of males in the litter, male-to-female ratio, or litter size. These results corroborate earlier findings of a sex difference in plasma testosterone levels on fetal day 19 in rats, and provide support for the hypothesis that female rats receive androgens from males located caudally in the uterus. No evidence was found that testosterone of pregnant females is affected by the sex ratio or size of her litter.

Males located caudally in the uterus affect sexual behavior of male rats in adulthood

It has been suggested that the sexual differentiation of female rats is affected by androgens from male fetuses in the uterine horn (intra-uterine position phenomenon). Effects of adjacent males, as well as of males located caudally in the uterus have been reported. The present study investigated whether male rats, like females, are affected by the presence of either caudal or adjacent male littermates. When tested in adulthood for sexual behavior, males that had male fetuses located caudally in the uterine horn showed shorter latencies to the first mount or intromission and shorter latencies to ejaculation, and exhibited more mounts and intromissions per minute than males that lacked caudal male siblings in the uterus. The presence of adjacent males did not significantly affect the parameters studied in this experiment.

Hormonally mediated inheritance of acquired characteristics in Mongolian gerbils

The intrauterine position relative to members of the same or opposite sex that a rodent fetus occupies affects both its morphology and behaviour when adult. Female fetuses that mature between males are androgenized by testosterone crossing fetal membranes, and their phenotypes as adults differ significantly from those of sisters that received less intrauterine exposure to exogenous testosterone. We report here that adult female Mongolian gerbils that gestated between male fetuses produce litters containing a significantly greater proportion of sons than the litters produced by those that gestated between female fetuses. Consequently, daughters delivered by dams that gestated between male fetuses are more likely to have gestated between male fetuses and be androgenized in utero than are daughters of dams that gestated between female fetuses. Female gerbils thus tend to inherit the phenotype (either androgenized or not androgenized) of their respective mothers.

Sensation seeking in opposite-sex twins: an effect of prenatal hormones?

Intrauterine hormones and position with respect to male and female littermates influence sexually dimorphic adult behavior in litter-bearing animals. Opposite-sex dizygotic twins offer the opportunity to examine analogous effects on sex-related human behaviors. To illustrate this approach, Sensation Seeking Scale (SSS) scores from 422 British twin pairs, including 51 opposite-sex pairs (Zuckerman, M., et al., J. Consult. Clin. Psychol. 46:139-149, 1978), were reanalyzed. Zuckerman et al. (1978) have shown that some aspects of sensation seeking are consistently increased in males relative to females. In comparing age-adjusted data for opposite and same-sex twins, our reanalysis demonstrated the predicted increase in sensation seeking in female members of opposite-sex pairs. Results were significant for measures of disinhibition, experience seeking, and overall sensation seeking. In contrast, male opposite-sex twins were not significantly different from male same-sex twins. Although psychosocial explanations of the increased sensation seeking in opposite-sex female twins cannot be excluded, these finding are consistent with hypothesized in utero hormonal influences on later behavioral development.

Blood flow in the uterine loop artery and loop vein is bidirectional in the mouse: implications for transport of steroids between fetuses

In rodents, steroids are able to pass between fetuses. Although not previously examined in mice, we have provided evidence that blood flow in the uterine loop artery in rats is bidirectional and that steroids are transported between fetuses by diffusion via the uterine lumen, not via the uterine blood vessels of the mother. The direction of blood flow in the uterine loop artery and vein feeding each uterine horn in house mice was examined on day 17 of pregnancy. Dye was injected into the heart to determine the direction of blood flow in the uterine artery while injection of dye into individual placentae was used to determine the direction of blood flow in the uterine vein. Blood entered the loop artery from both the dorsal and caudal ends and was thus bidirectional. Venus blood flow from placentae was in a rostral direction from placentae in the rostral portion of a uterine horn and in a caudal direction from placentae in the caudal portion of a uterine horn. Comparison of anogenital distance and body weight at birth using a variety of classification schemes, based on different assumptions about the mechanism and direction of transport of steroids between fetuses, showed that the only scheme which accounted for variation in anogenital distance at birth in female mice was that which was based on the number of directly adjacent male siblings in utero. Taken together with recent findings in rats reported elsewhere, we conclude that blood flow in the uterine blood vessels is bidirectional, and steroids are transported between fetuses by diffusing through the amniotic fluid and across the fetal membranes of adjacent fetuses.(ABSTRACT TRUNCATED AT 250 WORDS)

The intrauterine position phenomenon and precopulatory behaviors of house mice

The effects of intrauterine position on two sexually dimorphic, precopulatory, reproductive behaviors, were investigated in male and female mice of three different genetic strains. The two behaviors were: (i) urinary odor preference and (ii) ultrasonic mating vocalizations. In addition, anogenital distance was measured both at birth and later in adulthood as a morphological index of masculinization. The intrauterine positions of individual mice relative to male and female siblings were classified according to two different classification schemes based upon hypotheses of interfetal steroid transfer: (i) the contiguity hypothesis which is based on diffusion of steroids within the uterus and (ii) the transvasculature hypothesis which posits the movement of fetal steroids via the maternal vasculature. In contrast to expectations, intrauterine position defined by either the contiguity hypothesis or the transvasculature hypothesis, did not have a consistent effect on urinary odor preferences, ultrasonic mating vocalizations, or anogenital distance in male and female house mice.

Expression of the Raf-1 protein in rat brain during development and its hormonal regulation in hypothalamus

To study mechanisms involved in the sexual differentiation of the rat brain, the expression of the protein product of the proto-oncogene c-raf-1 (Raf-1) was examined. Biochemical and immunocytochemical analyses localized Raf-1 in embryonic rat brain regions and demonstrated hormonally induced changes in Raf-1 expression. For this study an affinity-purified anti-peptide antiserum specific for Raf-1 (NH-44) was used. Western blots revealed an approximately 77 kD polypeptide isolated in the cytosol of developing rat brains. Raf-1 levels were highest in the embryonic (E) day 22 female hypothalamus (HYP), and approximately twofold higher than levels detected in male HYP at E22 as determined by quantitative protein dot blot and semiquantitative Western blot analyses. Raf-1 levels in HYP were greater than those in either brain stem (BS) or cortex. Immunocytochemical analysis revealed high levels of Raf-1 in selective brain regions (e.g., the ventromedial nucleus in the HYP, the mitral cell layers in the main and accessory olfactory bulbs (OB), and the locus coeruleus) at E22 and postnatal (P) day 1. Lower levels of immunoreactivity were observed in many areas of the perinatal neuraxis. To test hormonal regulation of Raf-1, testosterone propionate (TP) was administered to pregnant rats on E17; male and female fetuses were examined on E22. This treatment significantly decreased Raf-1 levels in female HYP, but not in male HYP, as determined by Western blot analysis. No significant sex difference or response to prenatal hormone treatments were observed in either brain stem or cortex. No significant sex difference was noted postnatally, and administration of TP 3 h after birth did not change Raf-1 levels examined 24 h later. In summary, Raf-1 was localized within selective regions of the rat brain, and its expression was altered by exogenous prenatal hormonal stimulation. One role for Raf-1 in signal transduction may be to delimit hormonal critical periods in sexual differentiation of the brain.

Sex steroids at birth: genetic and environmental variation and covariation

Three sex-steroids (estradiol, progesterone, & testosterone) were assayed from the umbilical cord blood of 58 same-sex twin pairs in an investigation of the effects of sex, as well as genetic and environmental factors, on neonatal hormone levels. Although significant mean differences were found between boys and girls for both testosterone and progesterone, sex appeared to account for very little of the total variation for any of the hormones. Results showed that genetic influences significantly affected within-sex variation in both estradiol and progesterone levels, while variations in the intrauterine (shared twin) environment accounted primarily for differences in levels of testosterone. Moderate correlations were also found among the three hormones. Multivariate biometrical analyses revealed these relationships to be explained by an underlying general factor of nonshared environmental influences affecting all three hormones. Genetic factors appeared to be specific to each hormone rather than correlated across hormones. These results suggest not only that genes are operating at this early age, but also that maternal and other prenatal factors (e.g., placental effects, uterine position) have a significant role in variations of sex-steroids and possibly on later behaviors.

Differential hormonal modulation of brain antigens recognized by the AB-2 monoclonal antibody

The expression of monoclonal antibody AB-2 immunoreactivity is age- and sex-dependent in radial glia of developing rat hypothalamus and is regulated by prenatal exposure to gonadal steroids. In the present study, several proteins were recognized by AB-2 and were distributed selectively in subcellular fractions from neonatal hypothalamus (HYP), remaining forebrain (FB), and brainstem regions. Immunoblots revealed polypeptide bands in 3 major molecular weight classes: one at approximately 195 kDa in the cytosolic compartment; and two doublets at 220 kDa and 340 kDa in both microsomal and crude mitochondrial membrane fractions. The 220 kDa and 340 kDa doublets were also Triton-insoluble, suggesting a cytoskeletal association. The 195 kDa-AB-2-immunoreactive band was present in both Triton-soluble and insoluble fractions. AB-2 also recognized several acidic glycolipids extracted from postnatal rat brain regions on immunoblots following high performance thin layer chromatography. One of the bands from postnatal rat brain extracts migrated similarly to purified bovine brain sulfatide, which was also immunoreactive with AB-2. AB-2 immunoreactivity with proteins, polar lipids, and sulfatide suggests that the epitope is a carbohydrate present in multiple cellular compartments. AB-2 recognized the same molecular bands in males and females. Testosterone treatment selectively decreased the level of the 195 kDa AB-2-immunoreactive polypeptide. The 195 kDa AB-2-immunoreactive polypeptide possibly acts in radial glia in the determination of sexually dimorphic neurons in the preoptic area/hypothalamus.

Concentrations of sex steroid hormones in pregnant and fetal Mongolian gerbils

Sex steroid concentrations in the plasma of 24-day pregnant Mongolian gerbils (Meriones unguiculatus) and their male and female fetuses were measured using radioimmunoassays. It was found that, on Day 24 of gestation: (a) androgen levels were higher in those male fetuses developing adjacent to no female fetuses than in those male fetuses developing between two female fetuses and (b) androgen levels were higher in those female fetuses developing between two male fetuses than in those female fetuses with no immediate, male neighbours. Further, plasma taken from 24-day pregnant dams that had exhibited vaginal opening at a relatively early age had significantly lower androgen levels and significantly higher estradiol levels than did plasma taken from 24-day pregnant dams that had exhibited relatively late vaginal opening. The data provide direct evidence of hormonal mediation of previously described differences both in the morphology and reproductive biology of male and female adult gerbils as a function both of their fetal intrauterine locations relative to members of the other sex and of the age at vaginal introitus of their respective dams.

Masculinization and defeminization of female rats by males located caudally in the uterus

It is assumed that female rats are masculinized by the presence of males in the same uterine horn. Two hypotheses regarding the mechanism have been proposed: 1) interamniotic diffusion of androgens (contiguity hypothesis) and 2) transport of androgens via the bloodflow (caudal male hypothesis). This study was designed to test these hypotheses while taking into account two previously uncontrolled factors: hemihysterectomy of the mother during pregnancy and birth by caesarean section. Pregnant females were hemihysterectomized during pregnancy or left intact; pups were born naturally or through caesarean section. Position in utero was determined. In adulthood all females were ovariectomized and tested for mounting behavior before and during testosterone treatment and lordosis behavior during estradiol treatment. It was found that females with males located caudally in the same uterine horn were more masculinized and defeminized than females without such males. Adjacent males had no influence on the behavioral sexual differentiation of females. These results confirm the "caudal male hypothesis" rather than the "contiguity hypothesis." Hemihysterectomy during pregnancy prevented the "caudal male effect." Birth through caesarean section did not interfere with the caudal male effect.

Androgen and estradiol levels in plasma and amniotic fluid of late gestational male and female hamsters: uterine position effects

Using radioimmunoassay we have measured the plasma and amniotic fluid levels of androgen and estradiol in male and female hamster fetuses nearing parturition. On Days 14 and 15 of gestation (day of birth = Day 16), plasma levels of androgen are higher in males than females while estradiol levels are equal. Amniotic fluid levels of these hormones, while lower than plasma, reflect the difference in androgen and the similarity in estradiol between sexes. Uterine position analysis on Day 14 suggests that female siblings located caudally suppress amniotic fluid androgen and elevate estradiol levels of male siblings. Comparison of Day 18 gestation male and female rat amniotic fluid androgen to Day 14 hamsters reveals that male rats are bathed in high levels of androgen. Female rats have lower levels which are not different from those of male hamsters. Female hamsters are exposed to little androgen. Relevance to behavioral sexual differentiation and the display of adult behavior is discussed.

Prior intrauterine position influences body weight in male and female mice

In two longitudinal studies, intrauterine location of male and female Rockland-Swiss mice relative to fetuses of the same and opposite sex dramatically influenced body weight. In one study, body weight of males and females that were located in utero between two female fetuses (OM animals) or between two male fetuses (2M animals) was assessed from birth to the time of weaning (25 days of age). The body weights of 2M females were indistinguishable from those of OM and 2M males on all but a few of the 25 postnatal days of life. Also, 2M females were significantly heavier than OM females from Postnatal Day 6 onward, and 2M males weighed significantly more than OM males from Postnatal Day 19 onward. In a second study, food intake and body weight of animals from different intrauterine locations were examined from 25 to 120 days of age. Regardless of prior intrauterine position, males were always heavier than females. However, prior in utero location modulated body weight in that 2M females were significantly heavier than OM females and 2M males were reliably heavier than OM males. Intrauterine position effects were observed in the absence of any appreciable influence of this variable on levels of food intake. Taken together, the results suggest that prior in utero location may influence metabolic set points involved in the regulation of body weight and fat storage.

Singleton female mouse fetuses are subsequently unresponsive to the aggression-activating property of testosterone

Eight-day pregnant mice were surgically prepared so that they carried a single fetus which was delivered by cesarean section on Day 18, one day prior to term. Other animals were permitted to carry their entire complement of fetuses which also were delivered surgically. Their position within a uterine horn relative to one another was noted. Adult ovariectomized testosterone-treated females that had been situated in utero contiguous to no male fetuses attacked an olfactory bulbectomized adult stimulus male. In contrast, females that resided singly in the uterus were unresponsive to the hormone; they displayed no aggressive behavior. Masculinization, therefore, appears to occur in utero in the absence of contiguity to male fetuses.

Aromatase activity in the perinatal rat forebrain: effects of age, sex and intrauterine position

Sex and regional differences in aromatase activity were characterized in brains of rats sacrificed on embryonic days 18 (E18) and 20 (E20) and on postnatal days 2, 4 and 13 (D2, D4, D13). Aromatase activity was measured in vitro in homogenates of the hypothalamus/preoptic area (HPOA) and temporal lobe (TL) from individual rats, using [3H]19-hydroxyandrostenedione as substrate in the presence of NADPH. The apparent Km of aromatase for 19-hydroxyandrostenedione in TL at D4 was similar in males (34 nM) and females (22 nM,). Aromatase activity in the HPOA was highest prenatally (E18) and gradually declined to low levels by D13. Aromatase activity in the TL remained constant from E18 to D2, increased significantly on D4, and subsequently declined to low levels by D13. The level of aromatase activity was significantly greater in males than in females on E18 and D4 in the HPOA and on D4 in the TL. Differences in aromatase activity within regions of the HPOA were studied at E20 and D4. At both ages, the activity was highest in the preoptic area, lower in the anterior hypothalamus (AH), and lowest in the posterior hypothalamus. Aromatase activity was significantly higher in males than in females in the AH, but only on D4. The contiguity of males in utero was not correlated with aromatase activity in brain regions of adjacent female fetuses. The present results indicate that significant sex differences in aromatase activity exist in specific brain regions only at discrete times during perinatal development.(ABSTRACT TRUNCATED AT 250 WORDS)

Contiguity to males in utero affects avoidance responding in adult female mice

Female mice that had been situated in utero between two female fetuses displayed higher levels of active avoidance responding in adult life than females that had been located between two male fetuses and males for whom uterine position was without effect. Uterine position, therefore, influences acquired as well as species-typical behaviors.