Brain sexual differentiation results from the interaction of genetic and hormonal influences. This study used a unique agonadal mouse model to determine relative contributions of genetic and gonadal hormone influences in the differentiation of selected brain regions. SF-1 knockout (SF-1 KO) mice are born without gonads and adrenal glands and are not exposed to endogenous sex steroids during fetal/neonatal development. Consequently, male and female SF-1 KO mice are born with female external genitalia and if left on their own, die shortly after birth due to adrenal insufficiency. In this study, SF-1 KO mice were rescued by neonatal adrenal transplantation to examine their brain morphology in adult life. To determine potential brain loci that might mediate functional sex differences, we examined the area and distribution of immunoreactive calbindin and neuronal nitric oxide synthase in the preoptic area (POA) and ventromedial nucleus of the hypothalamus, two areas previously reported to be sexually dimorphic in the mammalian brain. A sex difference in the positioning of cells containing immunoreactive calbindin in a group within the POA was clearly gonad dependent based on the elimination of the sex difference in SF-1 KO mice. Several other differences in the area of ventromedial hypothalamus and in POA were maintained in male and female SF-1 KO mice, suggesting gonad-independent genetic influences on sexually dimorphic brain development.

Female receptivity including the immobile hormone-dependent lordosis posture is essential for successful reproduction in rodents. It is well documented that lordosis is organized during the perinatal period when the actions of androgens decrease the males’ ability to display this behavior in adulthood. Conversely the absence of androgens, and the presence of low levels of prepubertal estrogens, preserves circuitry that regulates this behavior in females. The current study set out to determine whether sex chromosomal genes are involved in the differentiation of this behavior. An agonadal mouse model was used to test this hypothesis. The SF-1 gene (Nr5a1) is required for development of gonads and adrenal glands, and knockout mice are consequently not exposed to endogenous gonadal steroids. Thus contributions of sex chromosome genes can be disassociated from the actions of estrogens. Use of this model reveals a direct genetic contribution from sex chromosomes in the display of lordosis and other female-typical sexual behavior patterns. It is likely that the concentrations of gonadal steroids present during normal male development modify the actions of sex chromosome genes on the potential to display female sexual behavior.

Rearing in social isolation has profound effects on several aspects of behavior in adult rodents. However, little is known about effects of social stress on social behavior in these animals. In the present study, we examined social recognition in mice of both sexes that were individually housed from 30 days of age until testing at approximately 80 days of age, individually housed from day 30 until day 60, followed by group housing from day 60 until testing at around 80 days of age and in control mice that were group housed throughout experiment. A standard social recognition test was performed with ovariectomized female conspecifics introduced into the home cage of tested mice for 1 minute, eight consecutive times with 9 minute breaks between tests, and in the ninth test, new, unfamiliar females were introduced. The time spent investigating stimulus mice during each of the nine tests was recorded. Group housed male and female mice showed strong pattern of social learning, whereas mice reared in isolation from day 30 until testing did not show evidence of social recognition. Interestingly, mice reared in isolation from 30 until 60 days of age and then group housed again, also showed reduced ability for social learning in comparison to the controls housed in groups through the entire period. These results therefore show that social isolation has a profound effect on social behavior in mice, and that even isolation for a limited period can produce lasting behavioral deficits.

Sex hormones are a major factor responsible for the development of sex differences. Steroidogenic factor 1 (SF-1) is a key regulator of gonadal and adrenal development, and SF-1 knockout mice (SF-1 KO) are born without gonads and adrenal glands. Consequently, these mice are not exposed to gonadal sex steroids. SF-1 KO pups die shortly after birth due to adrenal deficiency. In the present study, SF-1 KO mice were rescued by neonatal corticosteroid injections followed by adrenal transplantations on day 7-8 postnatally. Control mice received corticosteroid injections and were gonadectomized prior to puberty. Mice were observed interacting with ovariectomized hormone primed females and gonad-intact males. In the absence of sex steroid replacement, adult SF-1 KO mice were significantly more aggressive than control mice in tests with stimulus females. After testosterone treatment, control males displayed significantly more aggression towards male intruders than control female mice, or male and female SF-1 KO mice, suggesting a developmental role of gonadal hormones in the expression of aggressive behavior and affirming SF-1 KO mice as a behavioral model to investigate affects of fetal gonad deficiency.Keywords sexual differentiation; VMH; steroidogenic factor 1; sex steroids; aggression Steroidogenic factor 1 (SF-1), officially designated NR5A, was discovered as a regulator of the cytochrome P450 steroidogenic enzymes (Lala, Rice, & Parker, 1992;Morohashi et al., 1994). SF-1 gene expression is restricted to few locations (i.e., gonads, adrenals, pituitary, and the ventromedial nucleus of the hypothalamus [VMH]; Parker et al., 2002). It is an essential factor for gonadal and adrenal development as SF-1 knockout (KO) mice are born without gonads and adrenal glands and have male to female sex reversal (Ingraham et al., 1994;Luo, Ikeda, & Parker, 1994). The structure of the VMH (Dellovade et al., 2000;Ikeda, Luo, Abbud, Nilson, & Parker, 1995;Shinoda et al., 1995) and gene expression in pituitary gonadotropesCorrespondence concerning this article should be addressed to Gregor Majdic, Center for Animal Genomics, Veterinary Faculty, University of Ljubljana, Gerbiceva 60, SI-1000 Ljubljana, Slovenia. E-mail: Gregor.majdic@vf.uni-lj.si. is also markedly altered in SF-1 KO mice (Zhao et al., 2001). Adrenal transplantation rescues these mice so that they can survive to adulthood, and transplanted adrenal glands appear to function normally as corticosterone levels are indistinguishable between adult WT and KO mice (Majdic et al., 2002). With highly selective alterations in VMH organization and lack of developmental exposure to gonadal steroid hormones, SF-1 KO mice provide a potentially important tool for delineating the roles of gonadal hormones and the VMH in a variety of sexdependent aspects of physiology and behavior. NIH Public AccessIn the context of behaviors, male mice castrated on the day of birth display less intermale aggressive behavior than males castrated at PND 10 when treated with androgens in adult life...

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