Reproductive effects of prenatal exposure to 5-bromo-2'-deoxyuridine on male rats

Hypothalamic Monoaminergic Pathology in a Neurodevelopmental Rat Model Showing Prenatal 5-Bromo-2'-Deoxyuridine Treatment-Induced Hyperactivity and Hyporeproductivity

OBJECTIVE

Prenatal treatment of rats with 5-bromo-2'-deoxyuridine (BrdU) is a neurodevelopmental model showing hyperactivity and impaired sexual activity. Human neurodevelopmental disorders, such as autism, exhibit sex-related pathology, but sex-related neurodevelopment has not been fully investigated in this model. We conducted this study to facilitate the understanding of the pathophysiology of neurodevelopmental disorders.

METHODS

Pregnant rats received 50 mg/kg BrdU on gestational days 9-15. The tissue content of dopamine (DA), serotonin (5-HT), and their metabolites dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid were measured in male and female offspring at 3 weeks (juveniles) and 10 weeks (adults) of age.

RESULTS

Prenatally BrdU-treated rats had reduced DA metabolism or DA content in the hypothalamus from the juvenile through the adult period without sex differences, but sex-specific striatal DA abnormalities emerged after maturation. A reduction in 5-HT metabolism was measured in the hypothalamus without sex differences throughout development. Developmental alterations in the striatal 5-HT states were sex-dependent. Temporal changes in DA or 5-HT metabolism were found in the frontal cortex and midbrain.

CONCLUSION

The sex-specific influence of a genotoxic factor on the development of the DA and 5-HT systems was clarified in the hypothalamus and striatum. The results suggest that the observed sex dependence and region specificity are related to the pathology of social dysfunction in neurodevelopmental disorders.

Does 5-bromo-2'-deoxyuridine (BrdU) disrupt cell proliferation and neuronal maturation in the adult rat hippocampus in vivo?

5-Bromo-2'-deoxyuridine (BrdU) is frequently used as a mitotic marker in studies of cell proliferation. Recent studies have reported cytotoxic effects of BrdU on neural progenitor cells in embryonic and neonatal brains in vivo and in adult tissue studied in vitro. The present study was conducted to assess whether BrdU interferes with cell proliferation and neuronal maturation in the rat adult hippocampus in vivo. BrdU effects across a wide range of doses (40-480 mg/kg) on cell proliferation and the population of immature neurons in the adult hippocampus were investigated using immunohistochemical labeling methods for the cell cycle marker Ki67 and a marker for immature neurons, doublecortin. BrdU did not influence cell proliferation in the dentate gyrus or the population of immature neurons observed in the adult hippocampus relative to those observed in saline treated controls. Thus, in contrast with reports of deleterious effects of BrdU in embryonic and neonatal tissue and adult tissue studied in vitro, BrdU does not appear to have cytotoxic effects on proliferating hippocampal cells or immature neurons in vivo in rats.

Dopamine transporter density and behavioral response to methylphenidate in a hyperlocomotor rat model

Rats exposed prenatally to 5-bromo-2'-deoxyuridine (BrdU-rats) display hyperlocomotive activity, making them a possibly useful animal model for the study of attention deficit hyperactivity disorder (ADHD). Using this model, we investigated dopamine transporter (DAT) density and behavioral outcomes in BrdU-rats, some of which were also administered methylphenidate, a psychostimulant that is widely used for the treatment of ADHD. Pregnant rats were exposed to BrdU from gestational day 9 through 15. In male offspring, DAT densities in different regions of the striatum were quantified at three weeks of age. At seven weeks of age, locomotor, rearing and grooming behaviors were evaluated in an open-field setting, with or without methylphenidate treatment (1 mg/kg or 4 mg/kg). The results revealed no significant changes in striatal DAT densities in BrdU-rats compared with controls. Extreme hyperlocomotion of BrdU-rats was detected in the open-field environment, an effect that was exacerbated following treatment with the lower and higher dose of methylphenidate. Such increase in locomotor activity was observed only with the higher dose in control animals. In summary, degeneration of dopaminergic neurons in the terminal field was not detected in juvenile BrdU-rats, although adult animals displayed hyperactive behavior in a mildly stressful environment as well as hypersensitivity to a psychostimulant that facilitates dopaminergic neurotransmission.

Neuropathological examination of fetal rat brain in the 5-bromo-2'-deoxyuridine-induced neurodevelopmental disorder model

The majority of prior developmental neurotoxicity studies focused on postnatal subjects rather than on the fetus. In the present paper, we demonstrate the use of histological examination of fetal rat (embryonic day 16.5) brain serial sections, employing Nissl staining and microtubule-associated protein 2 (MAP2) immunohistochemistry, in evaluating a chemical-induced neurodevelopmental disorder. Since prenatal treatment with 5-bromo-2'-deoxyuridine (BrdU) is known to induce behavioral abnormalities such as locomotor hyperactivity in offspring, pregnant rats were administered 50 mg/kg on gestation days 9.5 through 15.5. The fetal brains at embryonic day 16.5 were collected and processed for neuropathological study. Cell death, including DNA strand breaks, was observed in specific areas of the fetal brain such as the neuroepithelium, intermediate zone and/or differentiating zones (e.g. neocortex and striatum) in exposed fetuses. In addition, the neocortex had an abnormal appearance cortical plate, which was also detected by MAP2 immunohistochemistry. The abnormal cortical plate was observed consistently, while the grade of cell death was generally very mild and variable. No significant alteration was detected in the brainstem. The present study reveals that histological observation of the fetal brain includes sensitive endpoints in developmental neurotoxicity, and that BrdU, at a dose generally administered to label proliferating cells, affects the development of the fetal neocortex.

Locomotor hyperactivity following prenatal exposure to 5-bromo-2'-deoxyuridine: neurochemical and behavioral evidence of dopaminergic and serotonergic alterations

Prenatal exposure to 5-bromo-2'-deoxyuridine (BrdU) has been reported to induce abnormal behaviors in offspring, including marked hyperactivity. In this study, the contribution of the serotonin (5-HT) and dopamine (DA) systems to BrdU-induced developmental neurotoxicity was investigated. Sprague-Dawley rats were treated with BrdU on gestational days 9 through 15 (50mg/kg, i.p.) and male offspring (BrdU-rats) were examined. The BrdU-rats exhibited a 3.5-fold increase in locomotor activity. The dopamine D2 receptor antagonist sulpiride increased locomotor activity in the BrdU-rats, but decreased it in control rats. The BrdU-rats responded to the 5-HT1A receptor antagonist NAN190 much more than the controls. The measurement of monoamines revealed significant decreases in DA, dihydroxyphenylacetic acid, and homovanilic acid, and significant increases in 5-HT and 5-hydroxy-3-indolacetic acid, with a decrease in the 5-HT turnover ratio in the striatum of BrdU-rats. Thus, prenatal exposure to BrdU induced alterations in both the DA and 5-HT systems.

Bromodeoxyuridine administered during neurogenesis of the projection neurons causes cerebellar defects in rat

Bromodeoxyuridine (BrdU) is broadly used in neuroscience to study embryonic development and adult neurogenesis. The potential toxicity of this halogenated pyrimidine analogue is frequently neglected. In this study, we administered BrdU in small doses by the progressively delayed cumulative labeling method to immunocytochemically tag different cerebellar cell types with antibodies to specific markers and BrdU in the same section. The well-known structure of the cerebellum made it possible to ascertain several toxic effects of the treatment. Time-pregnant rats were given five or six injections of 5 or 6 mg of BrdU ( approximately 12-20 mg/kg) at 8-hour intervals over 2 successive days between day 11 and 21 of pregnancy (E11-E12 to E20-E21), and the adult progeny was processed by immunocytochemistry. We demonstrate that this treatment effectively labeled distinct cerebellar cell populations but produced striking defects in the proliferation, migration, and settling of the Purkinje cells; reduced the size of the cerebellar cortex and nuclei; produced defects in the patterning of foliation; and also affected litter size, body weight, and mortality of the offspring. The observed toxic effects were consistent within individual treatment groups but varied between different treatment groups. Treatment with BrdU at the peak of neurogenesis of cerebellar projection neurons (E14) produced the most severe malformations. We observed no overt effects on the timing of neurogenesis for cerebellar neurons and glia across experimental groups. In conclusion, BrdU is a useful tool to study neural development, but its cytotoxicity represents a serious pitfall particularly when multiple doses are used to label cells.

Embryonic and postnatal injections of bromodeoxyuridine produce age-dependent morphological and behavioral abnormalities

The mitotic marker 5-bromodeoxyuridine (BrdU) was injected twice daily (60 mg/kg) into pregnant hooded rats on one of embryonic days (E) 11, 12, 13, 15, 17, or 21, or into rat pups on postnatal day (P) 10. The principal findings were the following: (1) BrdU exposure on E11 produces profound effects on body morphology, and animals must be fed a special diet because of chronic tooth abnormalities; (2) BrdU exposure at E17 or earlier produces a change in coat spotting pattern, the precise pattern varying with age; (3) BrdU exposure on E15 or earlier produces a reduction in both brain and body weight; (4) BrdU exposure on E17 or earlier reduces cortical thickness; (5) BrdU exposure on E11-E13 and at P10 reduces cerebellar size relative to cerebral size; (6) spatial learning is significantly affected after injections of BrdU at E11-E17, but the largest effect is on E17; (7) the deficit in spatial learning may be related in part to a reduction in visual acuity; and (8) skilled forelimb ability is most disrupted after BrdU exposure at E15 but is also impaired after injections on E13 or earlier. BrdU thus has teratological effects on body, brain, and behavior that vary with the developmental age of the fetus or infant.

Behavior and reproductive function of rat male offspring treated prenatally with 5-bromo-2'-deoxyuridine

5-Bromo-2'-deoxyuridine (BrdU) was administered intraperitoneally to Sprague-Dawley rats at doses of 50 mg/kg/d on Days 9 through 15 and at 100 mg/kg/d on Days 16 through 20 of gestation. Dams were allowed to deliver naturally. Male offspring were subjected to a variety of pre- and postweaning behavioral tests: surface righting, negative geotaxis, open field test, Biel maze test, wheel cage test, and shuttlebox avoidance test. After puberty, masculine sexual behavior was observed. Male offspring of dams treated with BrdU on Days 9 through 15 of gestation showed an accelerated negative geotaxis reflex and increased ambulation and rearing in open field, while those of dams treated on Days 16 through 20 of gestation showed normal activity. Offspring of dams treated on Days 9 through 15 of gestation showed a higher activity level in the wheel cage than offspring of dams treated on Days 16 through 20 of gestation. In the Biel maze, offspring of dams treated on Days 9 through 15 of gestation showed impaired learning and memory. In the shuttlebox avoidance response, offspring of dams treated on Days 9 through 15 of gestation moved significantly more than offspring of dams treated on Days 16 through 20 of gestation. Masculine sexual behavior was markedly reduced in male offspring of dams treated on Days 9 through 15 of gestation. However, no significant differences between groups in blood pressure nor heart rate were noted. We conclude that male offspring of dams treated with BrdU on Days 9 through 15 of gestation are hyperactive without hypertension and that these offspring show an impairment of masculine sexual behavior, i.e., hyposexuality.

Effects of prenatal exposure to 5-bromo-2'-deoxyuridine on the developing brain and reproductive function in male mouse offspring

The effect of prenatal exposure to 5-bromo-2'-deoxyuridine (BrdU) on the brain and reproduction in mice was studied. ICR mice were treated intraperitoneally (i.p.) with BrdU at 200 mg/kg on Day 10, 13, or 15 of gestation, or with BrdU at various doses (100 to 800 mg/kg) on Day 10 of gestation. In both experiments, dams were allowed to deliver, and male offspring were aged for 10 weeks and then cohabited with untreated females. In the phase-specificity study, the copulation rate was significantly decreased in the group treated on Day 10 of gestation, while the rate in the groups treated on Day 13 or 15 was comparable to the control level. In the dose-dependency study, copulation rates in the groups treated with BrdU at 200, 400, and 800 mg/kg were significantly lower than the control level, while the rate in the group treated with BrdU at 100 mg/kg was comparable to the control level. Masculine sexual behavior in the group treated with BrdU at 800 mg/kg was markedly impaired. Neither histopathologic changes of testis and sex-accessory glands nor alterations of sperm motility and concentration were observed in the offspring of the highest dose group. Dilatation of the third ventricles was observed in the highest dose group, whereas the relative brain weight in this group was comparable to that in the control group. In the subsequent study, ICR mice were treated i.p. with BrdU at various doses (25 to 800 mg/kg) on Day 10 of gestation, and the embryos were obtained 24 h after treatment. Histopathologic evaluation was performed in the ventricular zone of the telencephalon as well as ependymal and mantle layers of diencephalon (hypothalamus). The incidence of pyknotic cells in these areas was increased linearly with increasing BrdU dose and the incidence in the ependymal and mantle layers of the diencephalon was higher than that in the ventricular zone of the telencephalon. From these results, we conclude that damage to the central nervous system resulting from excessive cell death in the developing brain, particularly in the ependymal and mantle layers of the diencephalon (hypothalamus) may lead to reproductive dysfunction in postpubertal male offspring.

Effects of prenatal exposure to 5-bromo-2'-deoxyuridine on reproductive function in male mouse offspring

The effect of the halogenated pyrimidine analog 5-bromo-2'-deoxyuridine (BrdU), on reproductive functions of male mouse offspring treated prenatally was studied. BrdU was administered intraperitoneally to pregnant ICR mice at 100 mg/kg/d on days 8 through 13 of gestation, and at 100 or 200 mg/kg/d on days 14 through 18 of gestation. Dams were allowed to deliver naturally. Male offspring were aged for 10 weeks and then cohabited with untreated female mice for assessment of reproductive performance. Histopathologic examination of the testes and pituitary, sperm analysis, and determination of plasma testosterone concentrations of offspring at 12 weeks of age were performed. In the subsequent study, pregnant ICR mice were treated with 200 mg BrdU/kg on day 10, 13, or 15 of gestation. The embryos or fetuses were obtained from mothers from 6 to 48 h after treatment, and pyknotic cells in the ventricular zone of the telencephalon were counted. There was a significant decrease in body weight gain of offspring in all of the BrdU-treated groups. A marked decrease in copulation rate was noted in the male offspring of dams treated on days 8 through 13 of gestation, whereas no significant decreases in copulation and fertility rates were found in the male offspring of dams treated on days 14 through 18 of gestation. Neither histopathologic examination of testes nor sperm analyses revealed adverse effects of this compound, whereas cysts in the pars distalis of the pituitary were observed in the male offspring treated on days 8 through 13 of gestation. Dilatation of the lateral ventricles was also observed in male offspring at 12 weeks of age in the group treated on days 8 through 13 of gestation. The incidence of pyknotic cells in the ventricular zone of embryos was markedly increased 24 h after treatment on day 10 of gestation. These results indicate that the impaired fertility of the male offspring of dams treated with BrdU on days 8 through 13 of gestation may be due to the effects of BrdU exposure on central nervous system function that result in loss of libido rather than to the direct effects of this compound on the male reproductive organs.