Effect of prenatal uterine position on male and female rats sexual behavior

Early prenatal sex steroids and sex-typed play behavior at 4 years of age

During pregnancy, estrogens and testosterone influence brain development, resulting in sex-typical behavioral phenotypes. Prenatal testosterone exposure is associated with more male-typical behaviors in rodents, monkeys, and humans; however, few studies have examined the relationship between maternal sex hormones within the normal range and sex-dimorphic behaviors. In this study, we examined associations between prenatal estrogens and testosterone and sex-typical play in The Infant Development and the Environment Study (TIDES), a multicenter pregnancy cohort. We collected prenatal serum during the first trimester (mean=11.1 ± 2.6 weeks) and assessed child play behavior using the maternally completed Pre-School Activities Inventory (PSAI) at a mean age of 4.5 ± 0.3 years. This analysis includes mother-child pairs with complete data on hormones, play behavior, and covariates (n = 192 boys and 207 girls). No associations were seen between testosterone and PSAI scores in boys or girls or between estrogens and PSAI scores in boys. In girls, we observed an inverse relationship between feminine PSAI scores and both estradiol (E2) and estriol (E3) in multivariable linear regression analyses (E2: -0.11 [95% CI -0.20, -0.02]; E3: -0.44 [95% CI -0.83,-0.04]). Because the relationship between sex hormones and PSAI scores appeared nonlinear, we fit piecewise regression models to better fit the data and identify inflection points (point at which there is a significant change in slope). Piecewise regression analyses yielded inverse associations between masculine PSAI scores and estrone (E1) at values of E1 > 1340 pg/mL and E2 at values of E2 > 2870 pg/mL in girls. Further studies are needed to better understand the role of prenatal sex steroids on sexually dimorphic behavior.

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.

n-butylparaben exposure during perinatal period impairs fertility of the F1 generation female rats

Parabens are a class of preservatives widely used in the majority of personal care products, cosmetics, medicines, and food products. However, current literature suggests its plausible role as an endocrine disruptor, hence the present study was undertaken to delineate the effects of n-butyl paraben on perinatally exposed F1 female rats. F₀ dams were exposed subcutaneously to n-butylparaben from gestation day 6 (GD 6) to postnatal day (PND) 21 with doses of 10, 100, and 1000 mg/kg Bw/day in corn oil. The F1 female rats were monitored for pubertal development and sexual maturation through PND 30, 45, and 75; which were subsequently subjected to fertility assessment at PND 75. Perinatal exposure to n-butylparaben resulted in- This study documents impaired steroidogenesis and folliculogenesis might be the prime reason for the reduced fertility of F1 female rats. Hence, our study suggests that health monitors need to counsel potential females planning for pregnancy to avoid exposure to parabens.

The sex ratio of a gilt’s birth litter can affect her fitness as a breeding female

Lifetime reproductive performance and behaviour are established in utero driven by genetics and the steroidal milieu of the uterine environment. Developing males synthesise androgens that can be absorbed by females in the litter. Consequently, the proportion of males in a litter has the potential to affect both lifetime reproductive performance and behaviour of female littermates. Although reproductive potential is understandably important for gilt selection, behaviour is becoming progressively more important as group housing of sows increases. Aggression can be a cause for removal from the breeding herd, and can also negatively affect reproductive performance and herd longevity. This review covers existing evidence of masculinisation in rodents and sheep, the mechanism causing masculinisation, and the knowledge gaps surrounding masculinisation in pigs. Premature culling of gilts is an economic and animal welfare issue in the pork industry resulting in low retention of gilts and reduced productivity. The selection of gilts needs to be improved to incorporate both lifetime reproductive potential and behaviour. We determined that there is cause to investigate the effect of the birth litter sex ratio of a gilt on her suitability for selection into the breeding herd.

Sex Differences in Human and Animal Toxicology

Sex, the states of being female or male, potentially interacts with all xenobiotic exposures, both inadvertent and deliberate, and influences their toxicokinetics (TK), toxicodynamics, and outcomes. Sex differences occur in behavior, exposure, anatomy, physiology, biochemistry, and genetics, accounting for female-male differences in responses to environmental chemicals, diet, and pharmaceuticals, including adverse drug reactions (ADRs). Often viewed as an annoying confounder, researchers have studied only one sex, adjusted for sex, or ignored it. Occupational epidemiology, the basis for understanding many toxic effects in humans, usually excluded women. Likewise, Food and Drug Administration rules excluded women of childbearing age from drug studies for many years. Aside from sex-specific organs, sex differences and sex × age interactions occur for a wide range of disease states as well as hormone-influenced conditions and drug distribution. Women have more ADRs than men; the classic sex hormone paradigm (gonadectomy and replacement) reveals significant interaction of sex and TK including absorption, distribution, metabolisms, and elimination. Studies should be designed to detect sex differences, describe the mechanisms, and interpret these in a broad social, clinical, and evolutionary context with phenomena that do not differ. Sex matters, but how much of a difference is needed to matter remains challenging.

Sexual antagonism for testosterone maintains multiple mating behaviour

1. The persistence of multiple mating remains one of the fundamental questions in evolutionary biology. In theory, multiple mating is predicted to improve female fitness cumulatively through direct and/or genetic benefits. However, intra-locus sexual conflicts may potentially constrain or even eliminate these benefits owing to the gender load imposed by sexually antagonistic selection. 2. Here, we tested whether sexually antagonistic selection can maintain the variance in multiple mating behaviour of bank voles (Myodes glareolus) by manipulating the hormone testosterone through artificial selection in the laboratory. Among mammals, testosterone is a sexually dimorphic fitness-related trait under selection and is known to affect mating behaviour. We conducted mating trials in which females derived from family-based selection of testosterone were sequentially paired with four males of different testosterone profiles. 3. We show that artificial selection for high testosterone increased the mating rate of males, but clearly decreased the number of partners that females mated with (and vice versa). Because multiple mating was beneficial for the reproductive success of both sexes, as evidenced by the positive Bateman gradients, the divergent evolutionary interests of testosterone between the sexes can maintain this polygynandrous mating system. 4. Our results highlight how mating rate is concordantly selected in both sexes; however, it is largely influenced by testosterone, which is under sexually antagonistic selection. 5. This study is the first one to emphasise the direct and indirect effects of the endocrine system not only on reproductive physiology and behaviour but also for the evolution of genetic mating strategies in mammals.

Testosterone treatment of pregnant rabbits affects sexual development of their daughters

The intrauterine position (IUP) may affect the sexual development of a female fetus in litter-bearing mammals. Females gestating between two males (2M) develop into masculinized adults compared to females without contiguous males (0M). Fetuses are known to secrete different types and amounts of steroid hormones during the prenatal period, and diffusing steroids may affect sexual differentiation of their neighbors, too. Exposure of elevated testosterone (T) levels generally results in masculinized anatomy and behavior in females of several rodent species. Our recent study showed that IUP-dependent masculinization is also present in the domestic rabbit, which shows large variation in sex related traits as adults. The aim of the present study was to test if the IUP effect in rabbits can be induced by exogenous testosterone treatment. By administering different doses of testosterone propionate (TP) to pregnant rabbits, and following anatomical and behavioral development of their female offspring, we obtained dose-dependent increase both in the anogenital distance (AGD) and chin-marking behavior of prenatal T exposed females. The effects of treatment corresponded to the variation due to intrauterine position in our previous study, namely, exposure to T of known external origin resulted in similar tendencies of masculinization in rabbit females as the in utero proximity to male siblings. Our results suggest that the IUP effects may have similar physiological bases across different mammalian taxa including Lagomorphs.

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.

Female prairie vole mate-choice is affected by the males' birth litter composition

Experimental testing and retrospective examination of breeding records were used to examine the influence of sex composition and/or size of males' birth litters on female mate-choice. Sexually naïve female prairie voles (Microtus ochrogaster) avoided males derived from all-male litters, but showed no preference for, or aversion to, males from single-male litters or from more typical mixed-sex litters. Examination of the pregnancy status of females after two weeks of pairing with a male allowed us to estimate the probabilites of a pups' intrauterine position relative to siblings for various litter sizes. The typical prairie vole pup derived from a mixed-sex litter comprised of 4.4 pups, and had a 13% chance of being isolated from siblings in utero and a 22% chance of being between siblings in utero. Pups from single-sex litters tended to be larger at weaning than did pups from mixed-sex litters; however, male size did not influence female choice behavior. These results suggest that some aspect of the perinatal experience of prairie vole pups from single sex litters can influence social interactions later in life.

Intrauterine proximity to male fetuses affects the morphology of the sexually dimorphic nucleus of the preoptic area in the adult rat brain

Previous studies on polytocous rodents have revealed that the fetal intrauterine position influences its later anatomy, physiology, reproductive performance and behavior. To investigate whether the position of a fetus in the uterus modifies the development of the brain, we examined whether the structure of the sexually dimorphic nucleus of the preoptic area (SDN-POA) of rat brains accorded to their intrauterine positions. Brain sections of adult rats gestated between two male fetuses (2M) and between two female fetuses (2F) in the uterus were analysed for their immunoreactivity to calbindin-D28k, which is a marker of the SDN-POA. The SDN-POA volume of the 2M adult males was greater than that of the 2F adult males, whereas the SDN-POA volume of the 2M and 2F adult females showed no significant difference. This result indicated that contiguous male fetuses have a masculinizing effect on the SDN-POA volume of the male. To further examine whether the increment of SDN-POA volume in adulthood was due to exposure to elevated steroid hormones during fetal life, concentrations of testosterone and 17beta-estradiol in the brain were measured with 2M and 2F fetuses during gestation, respectively. On gestation day 21, the concentrations of testosterone and 17beta-estradiol in the brain were significantly higher in the 2M male rats as compared with the 2F male rats. The results suggested that there was a relationship between the fetal intrauterine position, hormone transfer from adjacent fetuses and the SDN-POA volume in adult rat brains.

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.

Behavioral deficits associated with fetal alcohol exposure are reversed by prenatal thyroid hormone treatment: a role for maternal thyroid hormone deficiency in FAE

Children prenatally exposed to alcohol typically exhibit behavioral abnormalities, including hyperactivity, learning deficits, and an increased prevalence of depression. Similar impairments are found in children of hypothyroid mothers, and we have shown that alcohol-consuming rat dams have suppressed hypothalamic-pituitary-thyroid (HPT) function. Therefore, we hypothesized that suppressed maternal thyroid hormonal milieu may contribute to the deleterious consequences of prenatal alcohol exposure. We aimed first to confirm and then to reverse the behavioral deficits in the fetal alcohol exposed (FAE) rat offspring by administration of thyroxine (T4) to the alcohol-consuming dams. Adult offspring prenatally exposed to ethanol (FAE; 35% ethanol-derived calories), pair-fed (PF) or control (C) diets were tested in the Morris water maze (MWM), the forced swim test (FST), and the open field test (OFT) to assess spatial learning, depressive behavior, and exploratory behavior/anxiety, respectively. Adult FAE offspring took longer to locate a hidden platform in the MWM and showed increased depressive behavior in the FST both of which were reversed by administration of T4 to the alcohol-consuming mother. We found sex and brain region-specific alterations in expression of genes involved in these behaviors in FAE adult offspring. Specifically, decreased hippocampal GAP-43 mRNA levels in adult FAE females and decreased glucocorticoid receptor (GR) expression in the amygdala of male and female FAE offspring were observed. The decreased mRNA levels of GAP-43 and GR were normalized by T4 treatment to the alcohol-consuming mother. Our results suggest that the suppressed HPT function of the alcohol-consuming mother contributes to the behavioral and cognitive dysfunctions observed in the offspring.

Long-lasting fitness consequences of prenatal sex ratio in a viviparous lizard

Maternal effects and early environmental conditions are important in shaping offspring developmental trajectories. For example, in laboratory mammals, the sex ratio during gestation has been shown to influence fitness-related traits via hormonal interaction between fetuses. Such effects have the potential to shape, or constrain, many important aspects of the organism's life, but their generality and importance in natural populations remain unknown. Using long-term data in a viviparous lizard, Lacerta vivipara, we investigated the relationship between prenatal sex ratio and offspring growth, survival, and reproductive traits as adults. Our results show that females from male-biased clutches grow faster, mature earlier, but have lower fecundity than females from female-biased clutches. Furthermore, male reproduction was also affected by the sex ratio during embryonic development, with males from male-biased clutches being more likely to successfully reproduce at age one than males from female-biased clutches. Thus, the sex ratio experienced during gestation can have profound and long-lasting effects on fitness in natural populations of viviparous animals, with important implications for life-history evolution and sex allocation.

Sexual incentive motivation, olfactory preference, and activation of the vomeronasal projection pathway by sexually relevant cues in non-copulating and naive male rats

There are some apparently healthy male rats that fail to mate after repeated testing with receptive females. We have previously shown that these "non-copulator (NC)" males show no partner preference for a receptive female when given the opportunity to physically interact with a sexually receptive female or a sexually active male. We also demonstrated that although NC males prefer odors from estrous females to odors from anestrous females, this preference is significantly reduced in comparison to the preference displayed by copulating (C) males. The aim of the present study was to evaluate in NC males sexual incentive motivation, that is, the approach behavior of male rats to either a sexually receptive female or a sexually active male in a test where the subjects can smell, hear, and see the stimulus animal but prevents their physical interaction. In addition, we determined whether NC rats have alterations in their ability to detect odors from conspecifics or odors related to food. In the detection of odors from conspecifics, we determined if these NC males are sexually attracted toward odors from receptive females or sexually active males. For food-related odors, we quantified the time it took the subjects to locate a hidden a piece of apple. Finally, using the induction of Fos-immunoreactivity (Fos-IR) as an index of neuronal activation, we compared the response of the vomeronasal projection pathway (VN pathway) of C and NC male rats exposed to estrous bedding. Males without sexual experience (WSE) were included in all experiments to determine the importance of previous heterosexual experience in the different behavioral tests and in the activity of the VN pathway. In the sexual incentive motivation test, we found that C and WSE male rats have a clear preference for estrous females over sexually active males, whereas NC male rats showed no preference. In odor tests, our results showed that C males had a clear preference for odors from estrous females as opposed to odors from sexually active males. Although NC and WSE male rats showed a preference for estrous female odors, this preference was significantly reduced compared to that shown by C males. No differences were found between WSE, C, and NC males in the detection of stimuli associated with food-related odors. A significant increase in Fos-IR was observed in the mitral cell layer of the accessory olfactory bulb in all groups when exposed to estrous bedding. However, only the C male rats exposed to estrous female bedding showed an increase Fos-IR in all structures of the VN pathway. An increase in Fos-IR was observed in the medial preoptic area (MPOA) of WSE males exposed to estrous bedding. No increases in Fos-IR were detected along the VN pathway in NC male rats. We proposed that NC male rats do not display sexual behavior due to a reduced sexual motivation that could be caused by alterations in the neuronal activity of the VN pathway during the processing of estrous odors.

Emotional reactivity in mice may not be inherited but influenced by parents

Heredity is often assimilated to genetic transmission of traits. However, some traits may be socially inherited. This has been described for maternal behaviour as well as for emotional reactivity in rodents such as rats or mice. The aim of the present study was to investigate further this idea using two backcrosses between CB6 or B6C females and C57BL/6 males. Indeed, the experimental groups are genetically identical but may be exposed to very different mothering types. When adults, the offspring were subjected to rodent emotional reactivity tests such as elevated plus maze and free exploration paradigm. Results show that CB6xB6 males exhibit higher emotional reactivity than B6CxB6 and B6 males in all behavioural situations, but these effects are not seen in females. Contrarily to their offspring that show different reactivity even if sharing the same genetical background, CB6 and B6C females display quite identical emotional reactivity. A possible explanation is that emotional reactivity is induced by maternal behaviour rather than transmitted by the parents.

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.

Steroid hormone modulation of olfactory processing in the context of socio-sexual behaviors in rodents and humans

Primer pheromones and other chemosensory cues are important factors governing social interactions and reproductive physiology in many species of mammals. Responses to these chemosignals can vary substantially within and between individuals. This variability can stem, at least in part, from the modulating effects steroid and non-steroid hormones exert on olfactory processing. Such modulation frequently augments or facilitates the effects that prevailing social and environmental conditions have on the reproductive axis. The mechanisms underlying the hormonal regulation of responses to chemosensory cues are diverse. They are in part behavioral, achieved through the modulation of chemoinvestigative behaviors, and in part a product of the modulation of the intrinsic responsiveness of the main and accessory olfactory systems to conspecific, as well as other classes, of chemosignals. The behavioral and non-behavioral effects complement one another to ensure that mating and other reproductive processes are confined to reproductively favorable conditions.

Risk-taking behavior in adolescent mice: psychobiological determinants and early epigenetic influence

Epidemiological research has emphasized that adolescence is associated with some temperamental and behavioral traits that are typical of this age and that might substantially contribute to both psychological and psychobiological vulnerability. The contribution of the important developmental rearrangements in neurobiological and neuroendocrinological processes has received surprisingly little investigation. The present review summarizes recent work in animal models, indicating that adolescent rodents exhibit marked peculiarities in their spontaneous behavioral repertoire. When compared to adults, adolescents show an unbalanced and 'extremes-oriented' behavior, consisting of an increased novelty seeking, together with decreased novelty-induced stress and anxiety, an increased risk-taking behavior in the plus-maze, as well as elevated levels of impulsivity and restlessness. Age-related discontinuities in the function of monoaminergic systems, which are a main target of abused drugs, can perhaps account for such a profile. In particular, a peculiar function within reward-related dopaminergic brain pathways actually seems to underlie the search for novel and rewarding sensations, as well as changes in the magnitude of psychostimulant effects. The role played by early epigenetic factors in the shaping of novelty-seeking behavior of adolescent and adult rodents are also reviewed. Two examples are considered, namely, subtle variations in the hormonal milieu as a function of intrauterine position and precocious or delayed maturation of nutritional independence as a function of changes in time of weaning. As for spontaneous drug consumption, a prominent vulnerability to the oral intake of nicotine during early adolescence is reported. In conclusion, adolescence in rodents may represent a suitable animal model with enough face- and construct-validity. Actually, this model is able to show behavioral features that resemble those found in human adolescents, including vulnerability to the consumption of psychoactive drugs.

Mammalian development in a changing environment: exposure to endocrine disruptors reveals the developmental plasticity of steroid-hormone target organs

Recent findings in the field of environmental endocrine disruption have revealed that developmental exposure to estrogenic chemicals induces morphological, functional, and behavioral anomalies associated with reproduction. The aim of the present study was to determine the effects of in utero exposure to low doses of the estrogenic chemical bisphenol A (BPA) on the development of the female reproductive tissues and mammary glands in CD-1 mice. Humans are exposed to BPA, which leaches from dental materials and plastic food and beverage containers. Here we report that prenatal exposure to BPA induces alterations in tissue organization within the ovaries and mammary glands and disrupts estrous cyclicity in adulthood. Because estrogen receptors are expressed developmentally in these estrogen-target organs, we propose that BPA may directly affect the expression of genes involved in their morphogenesis. In addition, alterations in the sexual differentiation of the brain, and thus the hypothalamic-pituitary-gonadal axis, may further contribute to the observed phenotype. The emerging field of endocrine disruptors promises to provide new insights into the mechanisms underlying the development of hormone-target organs and demonstrates that the environment plays important roles in the making of phenotypes.

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.

Emotional reactivity in mice, a case of nongenetic heredity?

Nongenetic heredity cases have been described in man, as well as in animals, and relationships between parents and offspring seem to play an important role in this transmission. In mice, mothering type could be nongenetically heritable by a latent learning close to mechanism. As mothering style clearly influences emotional reactivity, this reactivity could be nongenetically transmitted over generations. To clarify this question, the mother's influence on adult offspring reactivity must be established (whatever its basis, genetic, social or other). Thus, two reciprocal F1 hybrids (CB6 from a BALB/c mother and B6C from a C57BL/6 mother) have been compared using an ethological analysis in animal tests of emotional reactivity such as the free exploration paradigm and the light/dark box. First results show a sharp influence of the mother's strain and that suggests an effect of mothering style. The offspring from C57BL/6 mothers display less reactivity in the free exploration paradigm than the offspring from BALB/c mothers. In the light/dark box, no difference has been found between the two hybrids. Moreover, the mother's influence is greater in males than in females.

Novelty seeking in periadolescent mice: sex differences and influence of intrauterine position

In rodents, beside basic sex differences, a certain degree of within-gender phenotypic variation can also be provided in utero by hormones from adjacent fetuses. We investigated novelty-seeking behavior in two groups of male and female mice from know intrauterine position: 2M (between males) and 0M (between females). Subjects were assessed during periadolescence (postnatal days 33--43), an ontogenetic phase, which is characterized by an elevated expression of this novelty-seeking behavior. Periadolescent mice underwent a familiarization session for 3 consecutive training days with one side of a two-chamber apparatus. On testing day 4, the opening of a partition, which allowed mice to freely move from the familiar compartment to a novel one, produced an increased behavioral arousal in all animals. Marked sex differences were found, with females being in general more active than males, whereas the latter showed significantly higher levels of novelty seeking than females. Uterine position failed to affect the profile of novelty preference in females, whereas within the male group 2M subjects expressed a marked profile of novelty seeking. The differential titers of sex hormones reported to characterize the 0M and 2M condition early in fetal development are suggested to account for the individual variability in the seeking for novelty within the male group during puberty.