**Sexual Dimorphism in Monoamine Metabolism in BrdU-Treated Rats Showing Behavioral Dopamine Hypersensitivity: An Animal Model of Schizophrenia**

Katsumasa Muneoka and Makiko Kuwagata

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/55348

#### **1. Introduction**

80 Sexual Dimorphism

1988, p. 303–318.

[77] Kamiya T. Morphological and ethological adaptations of Ostracoda to microhabitats in *Zostera* beds. In: Hanai T., Ikeya N., Ishizaki K. (eds.) Evolutionary Biology of Ostracoda–Its fundamentals and applications. Tokyo: Kodansha, Amsterdam: Elsevier;

> A nucleotide analog 5-bromo-2'-deoxyuridine (BrdU) is a genotoxic compound that is incorporated into DNA [1]. When rodent fetuses are exposed to BrdU prenatally, the cortical development is profoundly affected. Cortical abnormalities induced by prenatal BrdU are shown as a reduction in thickness of the cerebral cortex [2]. An induction of apoptotic cell death in the mouse and rat fetal brain [3, 4] and disturbance to normal migration that induces abnormal composition of cortical glutamatergic or GABAergic neurons have been demonstrated [4]. Mouse data suggest that the prenatal BrdU treatment induced apoptotic phenomenon without sex difference (Figure 1).

> Adult rats that were prenatally treated with BrdU show locomotor hyperactivity. The hyperactivity was observed in both male and female rats and was characterized as an increase in spontaneous motor activity during dark cycles observed in home cages [5] and novelty-induced hyperlocomotion in the open-field [5 - 8]. This abnormal behavior is exacerbated by the treatment with dopamine (DA) agonist, methylphenidate, which indicates that animals acquired hypersensitivity to dopaminergic stimuli [5, 8]. Recently proposed animal models for schizophrenia that knock-out candidate genes for this disease, such as neurotensin receptors [9] or calcium/calmodulin-dependent kinase II alpha [10], show hyperactive phenotypes and abnormal striatal DA function while mice overexpressing DA D2 receptors in the striatum show unaltered locomotor activity [11]. In a major psychiatric disorder schizophrenia, disturbance in cortical development and subcortical dopaminergic abnormality have been proposed [12] and they induces hypothetical

© 2013 Muneoka and Kuwagata , licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

pathology in this disease "a prefrontal hypodopaminergia and a subcortical hyperdopaminergia" [13]. We propose that prenatal BrdU-treated rats is an animal model of schizophrenia based on findings that 1) the malformation of the cerebral cortex and 2) DA hypersensitivity.

Sexual Dimorphism in Monoamine Metabolism in BrdU-Treated Rats Showing Behavioral Dopamine Hypersensitivity: An Animal Model of Schizophrenia 83

steroid signaling has implications for both males and females with schizophrenia, as testosterone can be converted directly into estrogen by brain aromatase and the lack of

The changes in hormone levels that accompany sexual maturation in puberty are critically involved in the development of the monoaminergic system [20]. In animal studies, in addition to the well-known modulatory effects of estrogen or progesterone on DA neurotransmission or DA-related behavior [21 - 25], it has been reported that neuronal systems whose dysfunction mediates the emergence of psychotic-like behavior develops after the emergence of puberty, suggesting a role for gonadal hormones in the expression of pathological phenotypes [26, 27]. A study of rhesus macaques has demonstrated that intact adult animals display attenuated prepulse inhibition than animals given prepubertal castration [28]. In addition, results from studies of sexual dimorphic effects of prenatal stress in rats imply that the developing brain of female fetuses is less sensitive to maternal stress exposure than male ones, however, enhanced aggressive behavior or disturbed estrous cycle are observed [29]. Neonatal stress manipulation, maternal deprivation from pups, increases DA and 5-HT levels in the striatum in adulthood and the magnitude of changes are greater

In this chapter, we introduce the development of this animal model, their behavioral and neurochemical characters. Then, we evaluate recent obtained data of sexual dimorphic changes in DA and serotonin (5-HT) metabolisms in the prenatally BrdU-treated rats, and

Sprague-Dawley rats were purchased from Charles River Laboratories (Tsukuba, Japan). They were housed in metal cages in a room in which the temperature and relative humidity were controlled at 24 ± 1°C and 50 ± 5%, respectively. Lights were turned on from 0700 to 1900 h daily, and food and tap water were freely accessed. At 11 weeks of age, female rats were cohabited overnight with males. Females with sperm in their vaginal smears were regarded as pregnant, and were randomly assigned to the control or test groups. The day when the insemination was confirmed was designated as GD 0. BrdU (Sigma, St. Louis, MO) was suspended in 0.5% sodium carboxymethyl cellulose (CMC Na) and intraperitoneally administered to the test animals at 1300 h daily on GD 9 through 15. Females in the test group received a BrdU dose of 50 mg/kg, whereas control females subjected to the same regime received 0.5% CMC Na (5 ml/kg). The dosages were based on body weight on GD 9. The day of birth was designated postnatal day 0 (PND 0). On PND 1, each litter was reduced to eight animals; four males and four females. On PND 21, all offspring were weaned. At 10 weeks of age, one male and female animal obtained from an independent litter were sacrificed by decapitation and their brains were removed between 1600 and 1800 h. Each brain was subsequently dissected on ice. All tissues were stored at –80°C until the assay. We arranged other groups to investigate the effect of gonadectomy: the BrdU/GDX and BrdU/non-GDX

effects of gonadectomy that was performed during the prepubertal period.

**2.1. Animals, drug treatments and gonadectomy** 

functional estrogen receptors is found in both males and females with schizophrenia.

in males than females [30].

**2. Methods** 

**Figure 1.** Incidence of cell death in fetal mouse brains on the embryonic day 12 after the treatment with BrdU at the embryonic day 11 without indenfying sex. Apoptotic features were inspected under Nissl stain. Data were indicated as percents of numbers of apoptotic cells; C and B in parentheses indicates control and BrdU-treated animals. Horizontal bars indicate means. Significant increases in the numbers of apoptotic cell death were indicated as asterisks: \*, \*\*; P < 0.05, 0.01, respectively vs. controls. Results suggest that apoptotic cell death occurred similarly in fetal brains between males and females because values in % cell death in BrdU-treated brains in sample of mixed males and females scattered without clustering. FCx, the frontal cortex; CCx, the central cortex; PCx, the posterior cortex; St, the striatum; Hipp, the hippocmpus; A-Th, the anterior thalamus; P-Th, the posterior thalamus; A-Hy, the anterior hypothalamus; P-Hy, the posterior thalamus; AMY, the amygdala; MB, the mamillary body; Pretect, the pretectum; MT, the mecencephalic tegmentum; SN+VTA; The sunbtantia nigra + the ventral tegmental area; SC, the superior colliculus; IC; the inferior colliculus; Isth, the inthmus; Cereb, the cerebellum.

Sexual dimorphism in animal models of mental disorders is interested theme to be explored because sexual difference has been reported in clinical features in such disorders including schizophrenia. Emergence of abnormal symptoms in human mental illnesses has been known to be relevant to the beginning of puberty and this phenomenon appears to be influenced by gonadal hormones [14]. Gender differences in the age-of-onset and prevalence of mental disorders indicate an involvement of sex hormones in schizophrenia. Schizophrenia is more likely to emerge during adolescence or shortly afterwards and the first symptoms of schizophrenia emerge earlier in men than women [15]. This is more direct evidence for the involvement of sex steroids is the observation that adult women with schizophrenia have worse psychiatric symptoms and increased rate of relapse when estrogen levels are low in premenstrual period, postparturition or postmenopausal period [16]. Furthermore, estrogen has been shown to improve recovery from acute psychotic symptoms and to reduce both positive and negative symptoms of schizophrenia [17]. In adult males with schizophrenia testosterone levels are reduced and are inversely correlated with negative symptoms [18]. Altered expression of estrogen receptor alpha putatively rendering them sex-steroid unresponsive or insensitive has been reported [19]. Blunted sex steroid signaling has implications for both males and females with schizophrenia, as testosterone can be converted directly into estrogen by brain aromatase and the lack of functional estrogen receptors is found in both males and females with schizophrenia.

The changes in hormone levels that accompany sexual maturation in puberty are critically involved in the development of the monoaminergic system [20]. In animal studies, in addition to the well-known modulatory effects of estrogen or progesterone on DA neurotransmission or DA-related behavior [21 - 25], it has been reported that neuronal systems whose dysfunction mediates the emergence of psychotic-like behavior develops after the emergence of puberty, suggesting a role for gonadal hormones in the expression of pathological phenotypes [26, 27]. A study of rhesus macaques has demonstrated that intact adult animals display attenuated prepulse inhibition than animals given prepubertal castration [28]. In addition, results from studies of sexual dimorphic effects of prenatal stress in rats imply that the developing brain of female fetuses is less sensitive to maternal stress exposure than male ones, however, enhanced aggressive behavior or disturbed estrous cycle are observed [29]. Neonatal stress manipulation, maternal deprivation from pups, increases DA and 5-HT levels in the striatum in adulthood and the magnitude of changes are greater in males than females [30].

In this chapter, we introduce the development of this animal model, their behavioral and neurochemical characters. Then, we evaluate recent obtained data of sexual dimorphic changes in DA and serotonin (5-HT) metabolisms in the prenatally BrdU-treated rats, and effects of gonadectomy that was performed during the prepubertal period.
