Slov Vet Res 2010; 47 (4): 155-8 UDC 57.08:591.46:612.4:599.323.4
Review Article
BEHAVIORAL CHARACTERIZATION OF STEROIDOGENIC FACTOR-1 KNOCKOUT MICE
Neža Grgurevič1*, Stuart A. Tobet2, Gregor Majdič1,3
1Center for Animal Genomics, Veterinary faculty, University of Ljubljana, Gerbičeva 60, 1000 Ljubljana, Slovenia; 2College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO, 80523-1680, USA; 3Institute of Physiology, Medical School, University of Maribor, Slomškov trg 15, 2000 Maribor, Slovenia
Corresponding author, E-mail: neza.grgurevic@gmail.com
Summary: Sex differences in the brain are mainly caused by sex steroid hormones. Steroidogenic factor-1 is a key regulator of gonadal and adrenal development, and SF-1 knockout mice (SF-1KO) are born without gonads and adrenal glands and the region of the ventromedial nucleus of the hypothalamus (VMH) is reorganized. Due to the absence of gonadal hormones during embryonic development and alterations of the VMH, SF-1 KO mice provide an important model to study VMH dependent and hormonally independent sex differences in brain and behaviour. Initial studies did not show significant sex differences that could be attributed to sex chromosome effects, but provided interesting differences in aggressive and affiliative behaviours between genotypes, that may be attributable to the disrupted cellular organization of the VMH in SF-1 KO mice. However, further more focused behavioural studies also revealed some sex differences that could be also attributed to the action of sex chromosomes.
Key words: brain; sex differences; mouse; behaviour
Introduction
Differences in the brains between males and females have been observed in many levels of structure and function (1). One of the central questions is how and when these differences arise? Mammalian sexual differentiation begins with an expression of the Sry gene normally found on the Y chromosome and differentiation of the male gonads. After testes develop, circulating testosterone (T) leads to long-lasting changes in the structure of the male brain. It is now generally accepted that in rodents, the process of masculinization and defeminization is mediated by estradiol, derived from the local aroma-tization of T. In females, ovaries do not secrete significant levels of steroid hormones during prenatal development and many female brain characteristics develop in the absence of hormonal secretion (2). However, some studies suggest that development of female brain also requires active feminization and
Received: 13 September 2010
Accepted for publication: 15 November 2010
this process could be regulated by estradiol secreted from the ovaries between birth and puberty (3). Later in life, gonadal secretion of steroid hormones continues to induce less permanent sex differences via activational effects. These differences are sex-specific caused by ovarian and testicular secretions and could be eliminated by gonadectomy (2). In the last 10-15 years, a growing number of studies have shown that some sex differences in the brain could arise independently of gonadal hormones and puta-tively by different effects of genes, especially located on the X or Y chromosomes (4). Such sex differences, in mice, have been described in parental and aggressive behaviour (5), learning of habits (6), and sniffing and grooming of an intruder (7). In most behavioural studies it is difficult to distinguish between organizational and activational effects since gonad-ally intact mice are used (4). Therefore we used a novel model, agonadal mice with disruption of the gene coding for steroidogenic factor-1. Steroidogenic factor 1 (SF-1), officially designated NR5A, was initially discovered as a regulator of the cytochrome P450 steroidogenic enzymes (8). Subsequent studies
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have defined broader roles for SF-1 in development and function of the hypothalamus-pituitary-gonad-al axis. SF-1 KO mice are born without gonads and adrenal glands and have male to female sex reversal of secondary sex structures. The organization of the region that would normally contain the ventromedial nucleus of the hypothalamus (VMH) and gene expression in pituitary gonadotropes is also markedly altered in SF-1 KO mice (9). After neonatal corticosteroid injections and adrenal transplantation these mice can be studied in adulthood (10). Due to lack of gonadal hormones during embryonic development and alterations of the VMH SF-1 KO mice provide an important tool for delineating the roles of gonadal hormones and the VMH in a variety of sex dependent aspects of physiology and behaviour.
Behavioural analyses of SF-1 knockout mice
In a recent study SF-1 KO (genetically females and males) and WT mice were gonadectomized prior to puberty and tested for social behaviours. In these studies, no sex differences were found between chromosomally male and female (XX and XY) SF-1 KO mice in three different hormonal settings - in hormonally naive mice and in mice primed with either testosterone or estradiol and progesterone. Nevertheless, these studies provided interesting results with regard to aggression in both hormonally naive and testosterone treated SF-1 KO mice. Hormonally naive SF-1 KO mice, in particular females, were aggressive against intruder mice (11). However, this aggression was moderate aggression and could be possibly attributed to increased anxiety like behaviour in SF-1 KO mice that was also found in other testing paradigms (e.g., EPM, (12)) and another study using VMH specific SF-1 knockout mice model (13). Interestingly, testosterone treatment reduced the aggression shown by SF-1 KO mice and induced strong aggressive behaviour only in WT male mice (11). This confirmed that testosterone during a developmental period is needed to display proper intermale aggressive behaviour as reported previously (14). Aromatization of T could be the major factor responsible for development of adult intermale aggression since ERaKO mice rarely display aggressive behaviour against bulbectomized males (15). In contrast, ER0KO (16) and testicularly feminized mice (17) do show normal intermale aggression.
In contrast to data for rats (18), our studies have shown that neonatal hormonal exposure is not necessary for proper expression of sex related behaviors
in mice since both testosterone and estradiol/proges-terone primed SF-1 KO mice display both male and female sex behaviour, respectively. Although lordosis quality was not scored in our initial studies with EB+P primed mice, we can speculate from the copu-latory behaviour of male mice that SF-1 KO mice were less receptive than WT females and also that females were more receptive than males regardless of their genotype, and this was further confirmed in follow up studies (19). Lower receptivity of SF-1 KO mice could be due to the absence of gonadal hormones during development (3), or just as likely due to the altered organization of the VMH. It is well known that the VMH plays important role(s) in lordosis behaviour and, consistent with our findings, impaired lordosis was also found in CNS-specific SF-1 KO mice (20). So far it is not known if genetic factors contribute to defemini-zation of female sexual behaviour in males independently or in concert with gonadal hormones. Defemini-zation could be regulated through ER0, since higher lordosis quotient was observed in ER0KO males compared to the WT males (21). It would be interesting to establish whether SF-1 KO males are partially de-feminized and if this process is regulated through different Y linked genes, ER0, or dopaminergic systems since expression of tyrosine hydroxylase is regulated by the Sry gene in the midbrain (22). Further experiments will be needed to test these hypotheses.
Acknowledgements
This work was supported by NIH grant MH61376 (SA.T. and G.M.), ICGEB grant CRP SLO 06/02, ARRS (Slovenian research agency) grants P4-0053 and J7-2093 (G.M.).
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PROUČEVANJE OBNAŠANJA MIŠI BREZ GENA SF-1
N. Grgurevič, S.A. Tobet, G. Majdič
Povzetek: Spolne razlike v možganih nastanejo predvsem zaradi delovanja spolnih hormonov. Steroidogeni faktor 1 (SF-1) je dejavnik, ki uravnava razvoj spolnih žlez in nadledvične žleze. Miši brez gena SF-1 (angl. Knockout; SF-1 KO) se rodijo brez spolnih in nadledvičnih žlez in imajo spremenjeno strukturo ventromedialnega jedra hipotalamusa (VMH). Zaradi pomanjkanja spolnih hormonov med embrionalnim življenjem in zaradi spremenjenega jedra VMH predstavljajo miši SF-1 KO pomemben model za proučevanje hormonsko neodvisnih in od VMH odvisnih spolnih razlik v možganih in v obnašanju. Naše začetne raziskave niso pokazale spolnih razlik, ki bi jih lahko pripisali vplivu spolnih kromosomov, vendar pa so pokazale zanimive razlike v agresivnem in socialnem obnašanju med kontrolnimi mišmi in mišmi brez gena SF-1, ki bi jih lahko pripisali spremenjeni strukturi jedra VMH. Pri bolj usmerjenih raziskavah v določene tipe obnašanja pa smo ugotovili nekatere zanimive razlike med spoloma tudi pri miših brez gena SF-1, ki kažejo na vpliv spolnih kromosomov na spolno različen razvoj možganov.
Ključne besede: možgani; spolne razlike;miš; obnašanje