N-ethyl-N-nitrosourea-induced teratogenesis of brain in the rat. A cellular and cytoarchitectural analysis of the neocortex

Differential exposure to N-ethyl N-nitrosourea during pregnancy is relevant to the induction of glioma and PNSTs in the brain

Exposure to N-nitroso compounds (NOCs) during pregnancy has been associated with an increase in brain tumors in the progeny. This study investigated the brain tumorigenic effect of N-ethyl N-nitrosourea (ENU) after differential exposure of rats during pregnancy. Sprague Dawley rats were exposed to a single dose of ENU (80 mg/kg) in three different circumstances: 1) at first, second or third week of gestation; 2) at the 15th embryonic day (E15) in consecutive litters and 3) at E15 in three successive generations. Location and characterization of the offspring's brain tumors were performed by magnetic resonance imaging and histopathological studies. Finally, tumor incidence and latency and the animals' survival were recorded. ENU-exposure in the last two weeks of pregnancy induced intracranial tumors in over 70% of the offspring rats, these being mainly gliomas with some peripheral nerve sheath tumors (PNSTs). Tumors appeared in young adults; glioma-like small multifocal neoplasias converged on large glioblastomas in senescence and PNSTs in the sheath of the trigeminal nerve, extending to cover the brain convexity. ENU-exposure at E15 in subsequent pregnancies lead to an increase in glioma and PNST incidence. However, consecutive generational ENU-exposure (E15) decreased the animals' survival due to an early onset of both types of tumors. Moreover, PNST presented an inheritable component because progeny, which were not themselves exposed to ENU but whose progenitors were, developed PNSTs. Our results suggest that repeated exposure to ENU later in pregnancy and in successive generations favours the development of intracranial gliomas and PNSTs in the offspring.

Mechanisms of neurotoxicity induced in the developing brain of mice and rats by DNA-damaging chemicals

It is not widely known how the developing brain responds to extrinsic damage, although the developing brain is considered to be sensitive to diverse environmental factors including DNA-damaging agents. This paper reviews the mechanisms of neurotoxicity induced in the developing brain of mice and rats by six chemicals (ethylnitrosourea, hydroxyurea, 5-azacytidine, cytosine arabinoside, 6-mercaptopurine and etoposide), which cause DNA damage in different ways, especially from the viewpoints of apoptosis and cell cycle arrest in neural progenitor cells. In addition, this paper also reviews the repair process following damage in the developing brain.

Animal models

Epilepsy accounts for a significant portion of the dis-ease burden worldwide. Research in this field is fundamental and mandatory. Animal models have played, and still play, a substantial role in understanding the patho-physiology and treatment of human epilepsies. A large number and variety of approaches are available, and they have been applied to many animals. In this chapter the in vitro and in vivo animal models are discussed,with major emphasis on the in vivo studies. Models have used phylogenetically different animals - from worms to monkeys. Our attention has been dedicated mainly to rodents.In clinical practice, developmental aspects of epilepsy often differ from those in adults. Animal models have often helped to clarify these differences. In this chapter, developmental aspects have been emphasized.Electrical stimulation and chemical-induced models of seizures have been described first, as they represent the oldest and most common models. Among these models, kindling raised great interest, especially for the study of the epileptogenesis. Acquired focal models mimic seizures and occasionally epilepsies secondary to abnormal cortical development, hypoxia, trauma, and hemorrhage.Better knowledge of epileptic syndromes will help to create new animal models. To date, absence epilepsy is one of the most common and (often) benign forms of epilepsy. There are several models, including acute pharmacological models (PTZ, penicillin, THIP, GBL) and chronic models (GAERS, WAG/Rij). Although atypical absence seizures are less benign, thus needing more investigation, only two models are so far available (AY-9944,MAM-AY). Infantile spasms are an early childhood encephalopathy that is usually associated with a poor out-come. The investigation of this syndrome in animal models is recent and fascinating. Different approaches have been used including genetic (Down syndrome,ARX mutation) and acquired (multiple hit, TTX, CRH,betamethasone-NMDA) models.An entire section has been dedicated to genetic models, from the older models obtained with spontaneous mutations (GEPRs) to the new engineered knockout, knocking, and transgenic models. Some of these models have been created based on recently recognized patho-genesis such as benign familial neonatal epilepsy, early infantile encephalopathy with suppression bursts, severe myoclonic epilepsy of infancy, the tuberous sclerosis model, and the progressive myoclonic epilepsy. The contribution of animal models to epilepsy re-search is unquestionable. The development of further strategies is necessary to find novel strategies to cure epileptic patients, and optimistically to allow scientists first and clinicians subsequently to prevent epilepsy and its consequences.

Distinctive expression of midkine in the repair period of rat brain during neurogenesis: immunohistochemical and immunoelectron microscopic observations

Distinctive expression of midkine (MK) was observed during the repair period of fetal brain neuroepithelium. MK is a heparin-binding growth factor that occurs as a product of a retinoic acid-inducible gene, and has a molecular mass of 13 kDa. MK expression was examined immunohistochemically and by immunoelectron microscopy during a period of repair in developing rat brain at the neurogenesis stage. Injury was induced in rat fetuses by transplacental administration of ethylnitrosourea (ENU) on embryonic Day (E) 16, and histological changes were examined up to 48 hr thereafter (i.e., up to E 18). In normal rat fetuses, MK immunostaining was observed in the cytoplasm and radial and horizontal processes of all cells in the neuroepithelium (NE), subventricular zone (SV), and intermediate zone (IMZ). In ENU-administered brains, cells in the NE, SV, and IMZ were damaged severely, especially 16-24 hr after ENU administration. The remaining neuroepithelial cells, with the exception of those in M-phase and the tips of processes at the ventricular surface, were negative for MK immunohistochemistry 16-24 hr after the administration of ENU. Forty-eight hours after the administration, the cytoplasm and processes of cells in the NE, SV, and IMZ were MK immunopositive. Our previous data reported that the cell cycle of most NE cells is synchronized to the S-phase 16 hr after ENU administration and to the M-phase at 24 hr, and many NE cells were recovered 48 hr after ENU administration. The previous results taken together with the present results indicate that: (1) MK expression does not increase during the repair period of the NE, being different from adults; (2) MK expression is likely to be suppressed at S-phase according to the condition of the NE; and (3) MK expression is not essential for every cell cycle phase of NE cells; but (4) is necessary to maintain the M-phase of NE cells.

Ethylnitrosourea-induced apoptosis in primordial germ cells of the rat fetus

Ethylnitrosourea (ENU) is a simple alkylating agent. It induces gene mutations in fetal primordial germ cells (PGCs), and a high incidence of congenital malformations is also found in the offspring of male mice treated with ENU at the embryonic stage. It is also reported that decreases in the fertility rate and weights of the testis and ovary were found in the offspring from dams treated with ENU. In this study, we analyzed the occurrence of apoptotic cell death and the expression of p53 protein which is thought to play an important role in the DNA damage-induced apoptosis after administration of ENU to pregnant rats on day 13 of gestation to obtain a clue for clarifying the toxic effect of ENU on PGCs. Apoptotic cells increased in PGCs in fetal gonads from 3 h after treatment. The number of apoptotic PGCs peaked at 6 h and gradually decreased towards 24 h after treatment. On the other hand, p53-positive PGCs increased from I h after treatment, prior to the induction of apoptosis. The number of p53-positive PGCs peaked at 3 h and returned to the control level at 24 h after treatment. These results suggest that ENU induces apoptosis in rat fetal PGCs immediately after its administration to dams and excess cell death by apoptosis may have a close relation to the later occurrence of decreases in the fertility rate and gonadal weight. Moreover, a possible involvement of p53 is suggested in the ENU-induced apoptosis in PGCs.

Ethylnitrosourea induces apoptosis and growth arrest in the trophoblastic cells of rat placenta

Ethylnitrosourea (ENU), a well known alkylating agent, induces congenital anomalies in fetuses when it is administered to pregnant animals. In previous studies, we reported that ENU induced apoptosis and growth arrest in fetal tissues and organs immediately after its administration to pregnant rats. In the present study, we investigated the histopathological changes of the placenta after ENU administration to pregnant rats on Day 13 of gestation (GD13) to obtain a clue for clarifying the role of the placenta in the process of fetal developmental disability induced by genotoxic stress. Apoptotic cells increased and DNA-replicating cells decreased in the trophoblastic cells in the placental labyrinth zone of the ENU-treated group by 3 h after treatment. The number of apoptotic cells peaked at 6 h after treatment and returned to control levels at 48 h after treatment. The number of DNA-replicating cells reached minimum levels at 6 h after treatment and returned to control levels at 48 h after treatment. By immunohistochemistry, p53-positive signals were observed in trophoblastic cells in the labyrinth zone of the ENU-treated group from 3 to 6 h after treatment. Significant decreases in fetal and placental weights were observed in the ENU-treated group at 2 days (GD15) and 8 days (GD21) after treatment. A reduction in the thickness of the labyrinth zone was histopathologically significant in the ENU-treated group. These results indicate that ENU induces apoptosis and growth arrest not only in fetal tissues, but also in trophoblastic cells in the rat placental labyrinth zone, and these placental changes may have roles in the induction of fetotoxicity and teratogenicity of ENU. Moreover, a possible involvement of p53 in the induction of apoptosis and growth arrest is suggested.

Types and three-dimensional distribution of neuronal ectopias in the brain of mice prenatally subjected to X-irradiation

The types and three-dimensional distribution of neocortical ectopias following prenatal exposure to X-irradiation were studied by a histological examination and computer reconstruction techniques. Pregnant ICR mice were subjected to X-irradiation at a dose of 1.5 Gy on embryonic day 13. The brains from 30-day-old mice were serially sectioned on the frontal plane at 15 microns, stained with HE and observed with a microscope. The image data for the sections were input to a computer, and then reconstructed to three-dimensional brain structures using the Magellan 3.6 program. Sectional images were then drawn on a computer display at 240 microns intervals, and the positions of the different types of neocortical ectopias were marked using color coding. Three types of neocortical ectopias were recognized in the irradiated brains. Neocortical Lay I ectopias were identified as small patches in the caudal occipital cortex, and were located more laterally in the neocortex in caudal sections than in the rostral sections. Periventricular ectopias were located more rostrally than Lay I ectopias, and were found from the most caudal extent of the presumed motor cortex to the most caudal extent of the lateral ventricle. Hippocampal ectopias appeared as continuous linear bands, and were frequently associated with the anterior parts of the periventricular ectopias.

Teratologic studies on rat perinates and offspring from dams treated with ethylnitrosourea (ENU)

Ethylnitrosourea (ENU), a well known DNA alkylating agent, induces anomalies in the central nervous system (CNS), craniofacial tissues, limbs and male reproductive organs. Recently we clarified that excess cell death caused by apoptosis occurred in these organs and tissues of rat fetuses from dams treated with ENU at day 13 of gestation (GD13). In this study, we examined fetuses at GD21 and offspring at 10 weeks of age after ENU administration to pregnant rats at GD13 in order to clarify the relationship between ENU-induced apoptosis in the fetal tissues and teratogenicity of ENU. Severe intrauterine growth retardation was observed in the ENU group, and the body weight of the offspring in the ENU group was significantly lower than that of the control group throughout the experiment. In addition, a high incidence of microencephaly, ectrodactyly and curved caudal vertebrae was observed in the offspring from dams treated with ENU at GD13. Judging from the results of our previous and present studies, it was strongly suggested that ENU-induced apoptosis in rat fetal tissues may play an important role in the induction of anomalies in the corresponding tissues.

Neuropathological changes in the cerebrum of IUGR rat induced by synthetic thromboxane A2

IUGR was induced by maternal administration of synthetic thromboxane A2 (STA2) from the 13th day of gestation. Fetuses and neonates showed a markedly significant weight reduction. In E16 IUGR brain, no pathological abnormalities were found, but morphological changes appeared in the cortical plate of E18 IUGR brain. In E20 IUGR brain, ectopic clusters of differentiating cells cytologically mimicking neuroblasts were found in the neuroepithelial layer, but these abnormal clusters of cells in IUGR brain of late gestation were never observed in PN7. Morphometric analysis of coronal-sectional areas of the brain and cortical plate demonstrated that there were no differences between IUGR rats and controls in E16 and E18. These areas were, however, significantly reduced in E20 and PN7 growth-retarded rats compared with the control. Because the period of STA2 administration coincides with the neuro-developmental stage of cell migration and differentiation, reduction of the uteroplacental blood supply might cause a transient abnormal cytoarchitecture of the cerebral cortex resulting in brain growth retardation.

Distribution of neuronal heterotopiae following prenatal exposure to methylazoxymethanol

The three-dimensional distribution of neuronal heterotopiae induced in rat brains by prenatal exposure to the cytotoxic drug, methylazoxymethanol acetate, has been examined by computer reconstruction techniques. Three types of heterotopiae may be identified in mature rat brains exposed between E11 and E16: Layer I heterotopiae, periventricular heterotopiae, and hippocampal heterotopiae. The distributions of Layer I heterotopiae and periventricular heterotopiae show clear temporospatial gradients; such that with subsequent age of exposure, Layer I heterotopiae are situated progressively more medially, dorsally, and rostrally, and periventricular heteotopiae are situated progressively more rostrally. Periventricular heterotopiae are most extensive following exposure to the agent on E14. For both of these heterotopiae there is a characteristic pattern of distribution for each gestational age of exposure to the agent. By contrast, hippocampal heterotopiae, consisting of misplaced pyramidal neurons in subfields CA1 and CA2 of Ammon's horn, did not show significant changes in distribution with different ages of exposure to the drug. The significance of these temporospatial gradients for mechanisms underlying the production of the heterotopiae is discussed.

Cytoarchitectonic investigation of the rat spinal cord following ethylnitrosourea administration at different developmental stages

We examined the histological findings and cytoarchitectonic alterations in the rat spinal cord following matrix cell degeneration caused at different developmental stages, from neural plate formation through neuroblast generation. Ethylnitrosourea (ENU) 20 mg/kg body weight was administered transplacentally to the fetuses on the 10th embryonic day (E10) to 14th. The observations were made until the 21st postnatal day. Normally, mitoses were present scatteredly in the matrix cell layer of the neural plate or neural tube on E10 or E11, and gradually restricted to the dorsal portion of the alar plate as development occurred. The localization and number of degenerative cells as well as the site and degree of neuronal decrease in the completed dysgenetic spinal cord seemed to correlate with the topography and frequency of the mitoses in the matrix cell layer at the time of ENU administration. Disorder in the pattern of cytoarchitecture of neurons was not observed. The degree of hypoplasia of the white matter was proportional to the intensity of decrease of the spinal neurons. Aberrant myelinated fibers were not seen. No reactive gliosis, fibrosis or abnormal vascularization was observed at any time.

Correlations between the degree and type of forebrain malformations and the simultaneous neuro-oncogenic properties of ethylnitrosourea after diaplacental exposure in rats, alone and in combination with X-irradiation

Single and combined treatments were performed in rats on day 13 of gestation with either ENU or ENU subsequent to various X-irradiation doses between 0.5 and 1.5 Gy. At this time of gestation, developmental anomalies of the brain are still inducible by any of these treatments, in addition to neurocarcinogenic effects after ENU alone or in combination with X-irradiation. We looked for correlations between the degree of brain malformations still detectable in the adult animals and the simultaneous occurrence of brain tumors. These evaluations were based on a histopathological analysis regarding the type and degree of malformation residues, as well as the type and distribution pattern of the tumors (especially regarding gliomas) within the forebrain. Both after ENU and X-irradiation plus ENU-treatment, the occurrence of glioma in the offspring was positively correlated with the degree of brain dysplasia. This effect was not only restricted to the total glioma incidence but also confirmed by the higher glioma multiplicity in major dysplastic brains. Additionally, gliomas were preferentially located within the subependymal layer, which simultaneously was most severely affected by the teratogenic effects after prenatal treatment. Although forebrain dysplasia generally presents a significant predisposition for glioma inducibility, this oncogenic event is apparently strictly inversely related to a certain type of forebrain malformation, namely the occurrence of heterotopic neuronal nodules within the telencephalic roof. They emerge from "rosettes," which are typical radiation lesions occurring only after doses above 1.0 Gy. In none of the forebrains which still revealed rosette-residues in later life could a simultaneous occurrence of gliomas be observed. This explains not only the substantial decrease of glioma incidence after combined treatment with 1.0 and 1.5 Gy X-irradiation doses, but also the consistent glioma multiplicity despite the decrease of frequency.

Permanent alterations in the rat spinal cord following prenatal exposure to N-ethyl-N-nitrosourea

Pregnant rats between gestational stages E14-E22 were given a single injection of N-ethyl-N-nitrosourea (ENU). Pups born of these females were sacrificed 60 days after birth and their spinal cords examined qualitatively and quantitatively. Quantitative analysis involved measurement of spinal cord length and volume, estimation of neuron number, and the measurement of individual cell dendritic number and length. Cytoarchitecturally spinal cords appeared normal in all animals regardless of the age when they were exposed to ENU. Animals exposed during the latter portion of neurogenesis in the spinal cord (E14-E16) had significantly (p less than 0.05) reduced volumes of gray matter and reduced cell counts. Cellular analysis showed that all animals exhibited some stunting of dendritic length, although the number of dendritic branches was significantly (p less than 0.01) higher than normal for neurons of the intermediate gray and the substantia gelatinosa. Increase in the number of dendrites per cell suggests a mechanism of structural compensation by the surviving neuronal cells following their exposure to the teratogen.

An aberrant nucleus in the telencephalon following administration of ENU during neuroembryogenesis

Administration of ENU to rats during embryonic development caused the emergence of aberrant nuclei in the dorsal telencephalon. They were located in the corpus callosum, and were composed of pyramidal and stellate neurons. This suggested that the aberrant nuclei arose out of the neurons of cerebral cortex that had failed to migrate during embryogenesis. The aberrant nuclei were found predominantly in the animals receiving ENU on day 18 or earlier of gestation.