Generalized growth retardation in rats induced by prenatal exposure to methylazoxymethyl acetate

Role of Hydrazine-Related Chemicals in Cancer and Neurodegenerative Disease

Hydrazine-related chemicals (HRCs) with carcinogenic and neurotoxic potential are found in certain mushrooms and plants used for food and in products employed in various industries, including aerospace. Their propensity to induce DNA damage (mostly O⁶-, N⁷- and 8-oxo-guanine lesions) resulting in multiple downstream effects is linked with both cancer and neurological disease. For cycling cells, unrepaired DNA damage leads to mutation and uncontrolled mitosis. By contrast, postmitotic neurons attempt to re-enter the cell cycle but undergo apoptosis or nonapoptotic cell death. Biomarkers of exposure to HRCs can be used to explore whether these substances are risk factors for sporadic amyotrophic laterals sclerosis and other noninherited neurodegenerative diseases, which is the focus of this paper.

Assaying embryotoxicity in the test tube: current limitations of the embryonic stem cell test (EST) challenging its applicability domain

Testing for embryotoxicity in vitro is an attractive alternative to animal experimentation. The embryonic stem cell test (EST) is such a method, and it has been formally validated by the European Centre for the Validation of Alternative Methods. A number of recent studies have underscored the potential of this method. However, the EST performed well below the 78% accuracy expected from the validation study using a new set of chemicals and pharmaceutical compounds, and also of toxicity criteria, tested to enlarge the database of the validated EST as part of the Work Package III of the ReProTect Project funded within the 6th Framework Programme of the European Union. To assess the performance and applicability domain of the EST we present a detailed review of the substances and their effects in the EST being nitrofen, ochratoxin A, D-penicillamine, methylazoxymethanol, lovastatin, papaverine, warfarin, β-aminopropionitrile, dinoseb, furosemide, doxylamine, pravastatin, and metoclopramide. By delineation of the molecular mechanisms of the substances we identify six categories of reasons for misclassifications. Some of these limitations might also affect other in vitro methods assessing embryotoxicity. Substances that fall into these categories need to be included in future validation sets and in validation guidelines for embryotoxicity testing. Most importantly, we suggest conceivable improvements and additions to the EST which will resolve most of the limitations.

Animal models of intellectual disability: towards a translational approach

Intellectual disability is a prevalent form of cognitive impairment, affecting 2-3% of the general population. It is a daunting societal problem characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social and practical adaptive skills. Intellectual disability is a clinically important disorder for which the etiology and pathogenesis are still poorly understood. Moreover, although tremendous progress has been made, pharmacological intervention is still currently non-existent and therapeutic strategies remain limited. Studies in humans have a very limited capacity to explain basic mechanisms of this condition. In this sense, animal models have been invaluable in intellectual disability investigation. Certainly, a great deal of the knowledge that has improved our understanding of several pathologies has derived from appropriate animal models. Moreover, to improve human health, scientific discoveries must be translated into practical applications. Translational research specifically aims at taking basic scientific discoveries and best practices to benefit the lives of people in our communities. In this context, the challenge that basic science research needs to meet is to make use of a comparative approach to benefit the most from what each animal model can tell us. Intellectual disability results from many different genetic and environmental insults. Taken together, the present review will describe several animal models of potential intellectual disability risk factors.

Developmental neuropathology in DNT-studies—A sensitive tool for the detection and characterization of developmental neurotoxicants

Developmental neurotoxicity (DNT-) studies are the first reproduction toxicity studies for which an extended histopathological examination of developing structures is required by the current EPA and OECD guidelines. The morphological screening includes a macroscopic evaluation of the brain and nervous tissue, brain weight parameters, gross morphometry of the brain, neurohistological examinations and a quantitative analysis of major brain areas. This review is intended to give an overview about the needs according to guideline requirements, practical approaches for a successful developmental neuropathology and its preconditions and does include examples of background data on the value and functional meaning of morphological data. A selection of experimental data from literature is also presented in the light of their contribution for the understanding of important, neurodevelopmental disorders in humans.

Molecular networks perturbed in a developmental animal model of brain injury

Methylazoxymethanol (MAM) is widely used as a developmental neurotoxin and exposure to its glucoside (i.e., cycasin) is associated with the prototypical neurological disorder western Pacific ALS/PDC. However, the specific molecular targets that play a key role in MAM-induced brain injury remain unclear. To reveal potential molecular networks targeted by MAM in the developing nervous system, we examined characteristic phenotypic changes (DNA damage, cytoarchitecture) induced by MAM and their correlation with gene expression differences using microarray assays (27,648 genes). Three day-old postnatal C57BL/6 mice (PND3) received a single injection of MAM and the cerebellum and cerebral cortex of PND4, 8, 15, and 22 mice were analyzed. DNA damage was detected in both the cerebellum (N7-mGua, TUNEL labeling) and cerebral cortex (N7-mGua) of PND4 mice, but progressive disruption of the cytoarchitecture was restricted to the cerebellum. A majority (>75%) of the genes affected (cerebellum 636 genes, cortex 1080 genes) by MAM were developmentally regulated, with a predominant response early (PND4) in the cerebellum and delayed (PND8 and 15) in the cerebral cortex. The genes and pathways (e.g., proteasome) affected by MAM in the cerebellum are distinct from cortex. The genes perturbed in the cerebellum reflect critical cellular processes such as development (17%), cell cycle (7%), protein metabolism (12%), and transcriptional regulation (9%) that could contribute to the observed cytoarchitectural disruption of the cerebellum. This study demonstrates for the first time that specific genes and molecular networks are affected by MAM during CNS development. Further investigation of these targets will help to understand how disruption of these developmental programs could contribute to chronic brain injury or neurodegenerative disease.

Refractory atypical absence seizures in rat: a two hit model

Medically refractory seizure disorders in children usually have malignant neurodevelopmental outcomes and often are associated with the presence of congenital cortical dysplasias in the brain. To date, there are no animal models of these disorders by which to test hypotheses of pathogenesis or to screen novel drugs for antiepileptic activity. In rats, treatment with the antimitotic agent methylazoxymethanol acetate (MAM) on gestational day (G) 15 produces a neuronal migration disorder similar to the cortical dysplasias seen in human brain. We sought to produce chronic, recurrent, medically refractory seizures by administration of the cholesterol biosynthesis inhibitor AY-9944 (AY) during postnatal development in rats exposed prenatally to MAM. Prenatal MAM and postnatal AY treatments resulted in spontaneous, recurrent atypical absence seizures that were characterized by bilaterally synchronous slow spike-and-wave discharges (SWD) with a frequency of 6 Hz. The MAM-AY-induced seizures were refractory to ethosuximide, sodium valproate, and the GABABR antagonist CGP 35348, and were exacerbated by carbamazepine. Histological examination of brains from MAM-treated rats showed hippocampal heterotopias, in addition to atrophy and abnormalities of cortical lamination. The MAM-AY-treated rat represents a reproducible model of refractory atypical absence seizures in children with brain dysgenesis.

Excitability of CA1 neurons in the model of malformation-associated epilepsy

Experimentally induced heterotopia exhibit many of the anatomical features characteristic of cortical malformations in children with early-onset epilepsy. We used extracellular field potential recordings from the dorsal hippocampus of intact adult rats to determine whether the excitability of CA1 pyramidal cells was enhanced in rats with experimentally induced hippocampal dysplasia. Electrical stimulation of afferent fibers resulted in more robust population responses in the CA1 region of methylazoxymethanol (MAM)-treated rats vs the controls. The local population of CA1 pyramidal neurons was more excitable in the MAM-treated rat than in the control animals after synaptic activation. These results suggest that the excitability of the CA1 region in rats with hippocampal dysplasia is greater than that in control animals.

The prenatal methylazoxymethanol acetate treatment: a neurodevelopmental animal model for schizophrenia?

The prenatal methylazoxymethanol acetate (MAM) treatment has been proposed as a suitable model for the neurodevelopmental aspects of schizophrenia since the morphological abnormalities it induces in the brain are subtle and in line with most reports of neuropathology in schizophrenic brains. However, the functional aspects of this treatment have not been investigated with behavioural paradigms that are relevant for the psychopathology of the symptoms of schizophrenia. In the present study, we investigated the validity of the prenatal MAM treatment as a developmental model for schizophrenia with a prepulse inhibition of the acoustic startle reflex, latent inhibition, locomotor activity, and cognition and emotionality with freezing in fear conditioning paradigms. We have conducted two studies: in Study I, MAM was injected from E09 to E12, and in Study II MAM was administered at later stages in the embryonic development, from E12 to E15. Morphologically, the prenatal MAM treatment induced mild to severe reduction in brain weights and in the entorhinal cortex, prefrontal cortex and striatum volumes, the severity of the effects depending on the timing of administration. However, despite the morphological abnormalities induced by the MAM treatments, no behavioural deficits were observed in the MAM-treated animals when compared to Controls in prepulse inhibition, latent inhibition with the two-way active avoidance, and in the freezing paradigms. Therefore, due to the consistent lack of treatment effect observed in the present investigation, we conclude that the prenatal MAM treatment has no validity as a behavioural model for schizophrenia.

Current status of developmental neurotoxicity: an industry perspective

The chemical industry, along with the rest of society, shares the fundamental goal to protect the health and safety of children. Most of the existing testing programs for environmental chemicals primarily address the adult organism. Developmental neurotoxicity testing (DNT) studies are especially designed to address the specific risks of the developing nervous system. At time, DNT studies are not a regulatory requirement (US EPA) for all pesticides, however, these studies are recommended for and are now being required as Tier II studies for compounds which have shown evidence of neurotoxicity, endocrine modulation or in some cases developmental or reproduction toxicity. Laboratories, which are going to conduct this type of study, should have broad experience in reproduction toxicology and neurotoxicology studies. The DNT study includes a so called 'behavioral testing battery' to examine the development of autonomic, motor, sensory, and cognitive functions, where tests are performed at different time-points to cover important developmental stages of the nervous system. The development of the structure of the nervous system is carefully examined by neuropathological and morphometric examinations at two time points to detect morphologic abnormalities and growth defects. Critical issues concerning the conduct and interpretability of DNT study data include experience in the handling and logistics of large number of animals, historical control data, lack of scientific data of certain developing processes and uncertainties concerning the choice of the best methodical approaches.

Postnatal behavior in hatano high- and low-avoidance rats following prenatal exposure to low-dose methylazoxymethanol

The hypothesis that genetic factors influence behavioral effects was tested in rats exposed prenatally to methylazoxymethanol (MAM). We examined whether baseline behavior is an important factor influencing behavioral effects, and whether a behaviorally selected strain was useful for study of neurobehavioral teratology. Pregnant high- and low-avoidance animals (HAAs and LAAs) of the Hatano strain, selectively bred for high and low shuttlebox avoidance responses, respectively, were given an IP injection of a low dose of MAM (15 mg/kg) on day 14 of gestation. The offspring of these animals were subjected to behavioral tests for locomotor activity (running-wheel and open-field tests) and learning ability (Biel maze and shuttlebox avoidance tests). There were no significant effects of MAM on running-wheel activity or shuttlebox avoidance learning, whereas the number of errors in the Biel maze was increased in the MAM offspring of both strains. Interestingly, open-field activity of the MAM offspring was markedly decreased in LAAs but not in HAAs. Therefore, an additional experiment was performed to determine plasma levels of ACTH and corticosterone following open-field exposure. When compared to control offspring of the respective strains, plasma levels of ACTH and corticosterone were not altered by prenatal MAM treatment in LAAs. Instead, the MAM offspring in HAAs exhibited decreased ACTH levels in absence of behavioral alterations. These results demonstrated that prenatal exposure to low doses of MAM may alter postnatal behavior and endocrine response of the offspring, although to a differing degree in HAAs and LAAs. Our observations suggested that behaviorally selected strains are sensitive to neurobehavioral teratogens such as MAM.

Characterization of heterotopic cell clusters in the hippocampus of rats exposed to methylazoxymethanol in utero

Cortical disorganization represents one of the major clinical findings in many children with medically intractable epilepsy. To study the relationship between seizure propensity and abnormal cortical structure, we have begun to characterize an animal model exhibiting aberrant neuronal clusters (heterotopia) and disruption of cortical lamination. In this model, exposing rats in utero to the DNA methylating agent methylazoxymethanol acetate (MAM; embryonic day 15) disrupts the sequence of normal brain development. In MAM-exposed rats, cells in hippocampal heterotopia exhibit neuronal morphology and do not stain with immunohistochemical markers for glia. In hippocampal slices from MAM-exposed animals, extracellular field recordings within heterotopia suggest that these dysplastic cell clusters make synaptic connections locally (i.e. within the CA1 hippocampal subregion) and also make aberrant synaptic contact with neocortical cells. Slice perfusion with bicuculline or 4-aminopyridine leads to epileptiform activity in dysplastic cell clusters that can occur independent of input from CA3. Taken together, our findings suggest that neurons within regions of abnormal hippocampal organization are capable of independent epileptiform activity generation, and can project abnormal discharge to a broad area of neocortex, as well as hippocampus.

Damage and repair of nerve cell DNA in toxic stress

It is generally agreed that ALS/PDC is triggered by a disappearing environmental factor peculiar to the lifestyle of people of the western Pacific (i.e., Guam, Irian Jaya, Indonesia, and the Kii Peninsula of Japan). A strong candidate is the cycad plant genotoxin cycasin, the beta-D-glucoside of methylazoxymethanol (MAM). We propose that prenatal or postnatal exposure to low levels of cycasin/MAM may damage neuronal DNA, compromise DNA repair, perturb neuronal gene expression, and irreversibly alter cell function to precipitate a slowly evolving disease ("slow-toxin" hypothesis). In support of our hypothesis, we have demonstrated the following: 1. DNA from postmitotic rodent central nervous system neurons is particularly sensitive to damage by MAM. 2. MAM reduces DNA repair in human and rodent neurons, whereas DNA-repair inhibitors potentiate MAM-induced DNA damage and toxicity in mature rodent nervous tissue. 3. Human neurons (SY5Y neuroblastoma) that are deficient in DNA repair are susceptible to MAM-induced cytotoxicity and DNA damage, whereas overexpression of DNA repair in similar cells is protective. 4. MAM alters gene expression in SY5Y human neuroblastoma cells and, in the presence of DNA damage and reduced DNA repair, enhances glutamate-modulated expression of tau mRNA in rat primary neurons; the corresponding protein (TAU) is elevated in ALS/PDC and Alzheimer's disease. These findings support a direct relationship between MAM-induced DNA damage and neurotoxicity and suggest the genotoxin may operate in a similar manner in vivo. More broadly, a combination of genotoxin-induced DNA damage (via exogenous and/or endogenous agents) and disturbed DNA repair may be important contributing factors in the slow and progressive degeneration of neurons that is characteristic of sporadic neurodegenerative disease. Preliminary studies demonstrate that DNA repair is reduced in the brain of subjects with western Pacific ALS/PDC, ALS, and Alzheimer's disease, which would increase the susceptibility of brain tissue to DNA damage by endogenous/exogenous genotoxins. Interindividual differences in the extent of prior exposure to DNA-damaging agents and/or the efficiency of its repair might produce population variety in the rate of damage accumulation and explain the susceptibility of certain individuals to sporadic neurodegenerative disease. Studies are underway using DNA-repair proficient and deficient neuronal cell cultures and mutant mice to explore gene-environment interplay with respect to MAM treatment, DNA damage, and DNA repair, and the age-related appearance of neurobehavioral and neuropathological compromise.

The Guam cycad toxin methylazoxymethanol damages neuronal DNA and modulates tau mRNA expression and excitotoxicity

As in Alzheimer's disease, brains of Guam Chamorros with amyotrophic lateral sclerosis (ALS) and Parkinsonism-dementia complex (PDC) contain intraneuronal-paired helical filaments composed of accumulated phosphorylated tau protein. Tau mRNA expression in rat neuronal cultures-normally modulated by glutamate-increases after treatment with the aglycone of cycasin, a cycad-derived toxin whose concentration in Chamorro food varies with disease incidence. Elevated Tau gene expression in vitro is coincident with increased cycasin-related DNA adducts and reduced DNA repair. Cycasin and endogenous glutamate may together promote the accumulation of tau protein and neuronal degeneration in Western Pacific ALS/PDC.

Methylazoxymethanol-induced micrencephaly in the brown Norway strain: behavior and brain weight

A single injection of 20 mg/kg methylazoxymethanol acetate (MAM) on gestational day 14 in Brown Norway rats produced micrencephalic offspring (whole brain approximately equal to 65% of control). Despite the micrencephaly, MAM-induced alterations in behavior assessed here were relatively mild. The MAM-treated rats exhibited increased activity under darkened conditions in a complex maze and marginally increased activity after a challenge of methamphetamine. Open field activity, running wheel activity, and emergence behavior using a light/dark apparatus were not significantly affected. Compared with a similar study of Sprague-Dawley micrencephalics [Ferguson S.A., Racey F.D., Paule M.G. and Holson R.R. (1993) Behavioral effects of methyloxymethanol-induced microencephaly. Behav. Neurosci. 107, 1-101], frontal cortex and striatum weights were more reduced in Brown Norway micrencephalics. The MAM-induced behavioral alterations in the Brown Norway strain may have appeared attenuated compared to alterations shown by MAM-treated Sprague-Dawley rats due to differences in baseline between these two strains. Compared to control Sprague-Dawley rats in the previous study, control Brown Norway rats were more active in the open field and running wheels, but less active in the complex maze, exhibiting little to no learning. Emergence tests indicated increased dark preference in Brown Norway rats. Baseline behavior (increased activity and light shyness) of control Brown Norway rats was similar to that of MAM-treated Sprague-Dawley rats; a potential confound in the detection of behavioral effects of a compound. These findings emphasize the effects that strain selection may have on the outcome and interpretation of toxicological/teratological studies.

Flurothyl seizure susceptibility in rats following prenatal methylazoxymethanol treatment

Methylazoxymethanol acetate (MAMac) is a potent teratogenic agent which can produce ectopic cell placement in developing rat brains. In the present study, we evaluated (i) whether prenatal exposure to MAMac results in a lowered seizure threshold to flurothyl and (ii) if there is a correlation between the number of ectopic cells in MAMac-exposed hippocampus and flurothyl-induced seizure latency. In 60 day old (P60) rats exposed to MAMac in utero, the latencies to myoclonic jerk (173 +/- 2.3 s) and forelimb clonus (215 +/- 4.6 s) were significantly shorter than those of controls (200 +/- 6.9 s and 238 +/- 8.8 s, respectively). MAMac also increased the proportion of flurothyl-treated rats that progressed from bilateral forelimb clonus to generalized tonic-clonic seizures (control: 33%; MAMac: 91%). Shorter seizure latencies were associated with an increased number of ectopic pyramidal cells in region CA1/CA2. These results suggest seizure susceptibility is enhanced in an animal model (MAMac) characterized by abnormal neuronal migration.

Electrophysiology of CA1 pyramidal neurons in an animal model of neuronal migration disorders: prenatal methylazoxymethanol treatment

Prenatal methylazoxymethanol acetate (MAMac) injection disrupts cell migration in developing rats. We investigated the electrophysiological characteristics of hippocampal CA1 pyramidal neurons from young MAMac-treated animals (postnatal days 25-35). In vitro intracellular recordings from CA1 cells in MAMac-treated tissue revealed resting membrane potential (mean, -61.5 +/- 1.5 mV), action potential amplitude (mean, 69 +/- 3.1 mV), action potential duration (mean, 2.1 +/- 0.2 ms), input resistance (mean, 51.5 +/- 3.6 M omega) and time constant (mean, 33.2 +/- 1.2 ms) similar to those of CA1 cells from control tissue. However, MAMac-treated tissue could be distinguished as having a higher percentage of cells (62% vs. 10%) which fire a burst of action potentials in response to suprathreshold current injection. The synaptic responses of CA1 cells in MAMac-treated and control tissue were comparable. The CA1 field response to stimulation was also comparable at all stimulus intensities tested (50-1500 microA). Elevation of extracellular potassium concentration ([K+]o) from 3 mM to 6 mM resulted in epileptiform discharge activity in response to stratum radiatum stimulation in all MAMac-treated slices (10/10) but in only one-third of controls (3/9). Spontaneous epileptiform discharges were also observed in the majority (8/13) of MAMac-treated slices bathed in 6 mM KCl but in no controls. These data suggest that MAMac treatment during fetal development not only disrupts normal anatomical organization but also leads to alterations in electrophysiological features of the hippocampal CA1 pyramidal cell region. As such, the MAMac model may provide insights into early onset seizure syndromes associated with developmental abnormalities.

Seizure susceptibility in immature rats with micrencephaly induced by prenatal exposure to methylazoxymethanol acetate

The administration of the alkylating neurotoxin methylazoxymethanol acetate (MAM) to pregnant rats on day 15 of gestation induces, in the offspring, a marked micrencephaly, characterized by an impaired formation of interneurons at cortical, hippocampal and striatal levels. Since in man developmental CNS malformations are often associated with severe epileptogenic encephalopathies with seizures appearing in the first months or years of life, we have studied the development of kainic-acid- and bicuculline-induced seizures in 15- and 30-day-old rats, prenatally exposed to MAM. Compared to controls, a higher susceptibility to seizures has been found in micrencephalic rats aged 15 days, while no significant differences have been observed in those aged 30 days. It is hypothesized that the cerebral global anatomical dysgenesis caused by MAM underlies the higher seizure susceptibility shown by animals during the first periods of life. Successively, the processes of adjustment occurring between the cerebral regions affected by the neurotoxic action of MAM and the afferent and efferent pathways spared by the substance may re-establish adequate interneuronal relationships and, therefore, a normal convulsive susceptibility.

Patterns of growth deficiency in rats exposed in utero to undernutrition, ethanol, or the neuroteratogen methylazoxymethanol (MAM)

Children and experimental animals exposed to ethanol (EtOH) in utero commonly have low birthweights, and many remain small at maturity. Low body weight or small stature in adulthood may reflect an inability to recover from in utero growth retardation, or it may reflect a separate, postnatal growth deficiency. In this study, daily body weights (postnatal days 1 to 60) were compared among the offspring of the following groups of Long Evans rats: dams fed liquid diet containing 35% EtOH-derived calories; their pair-fed and chow-fed controls; and dams exposed to methylazoxymethanol (MAM) in two previous studies, in which offspring exhibited reduced numbers of growth hormone releasing factor (GRF) neurons. All treatments produced a number of offspring with weight deficits beginning after birth and persisting into maturity. Three distinct patterns of growth deficiency were observed: (1) weight loss relative to controls in the first weeks of life, seen in offspring exposed to EtOH, pair feeding, or MAM on gestation day 13 (G13); (2) a delay in the onset of the prepubertal growth spurt, seen in all EtOH-exposed offspring and in G13 MAM-exposed dwarfs; and (3) failure to sustain the prepubertal growth spurt, seen only after exposure to MAM on G14. The results of this study support the view that prenatal EtOH exposure is capable of affecting postnatal growth specifically; moreover, the pattern of growth deficiency seen in EtOH-exposed offspring was distinct from that of the undernourished offspring of pair-fed dams.

Premature decline in Morris water maze performance of aging micrencephalic rats

The rat with methylazoxymethanol-induced micrencephaly is a useful animal model of congenital brain defects and associated cognitive impairment. Born with profound morphological and neurochemical alterations in the forebrain, it shows impaired ability to learn mazes. In order to determine how an animal with such a developmentally damaged brain would function in old age, Long-Evans rats 6, 15, and 24 months of age were tested for their ability to learn to locate a hidden platform in the Morris water maze. The performance of micrencephalic rats of all ages was impaired on acquisition, retention, and transfer trials. Moreover, the magnitude of their acquisition deficit increased with age. It remains to be determined whether the premature decline of the micrencephalic rat in learning the task simply reflects a greater impact on an already compromised brain by neuron loss characteristic of aging brains or whether the prenatal insult alters some basic processes resulting in premature aging.

The relationship of rat brain weight and pituitary weight to postnatal growth after prenatal exposure to methylazoxymethanol

Teratogens can affect body weight in various ways, but the association of brain damage with postnatal growth abnormalities suggests a role for neuroendocrine growth-controlling systems. Growth deficiencies follow methylazoxymethanol (MAM) exposure during the period when the growth hormone releasing factor (GRF) cells of the hypothalamus form, and the pattern of growth of the animals is like that of animals deficient in growth hormone. The present studies were designed to examine the growth, body proportions, brain weight, and pituitary weight of animals treated with 20 mg/kg MAM on the 13th day of gestation, a peak period for production of GRF neurons. Among the offspring, this treatment produced about 25% dwarfs (animals smaller than the smallest control of the same sex). Significantly more females than males were categorized as dwarfs. The weight effect occurred long after birth, as is characteristic of animals and humans with growth hormone deficiency. Analyses of weights over the course of development indicated that prenatal factors, rather than factors operating between birth and weaning, predicted the adult body weight of dwarfs, while both sets of factors were significant in other animals. The growth reduction was symmetrical, as would be expected if the animals were growth hormone deficient, with an 18% reduction in weight reflecting a 6% reduction in bone length. The remaining treated animals were similar to controls in absolute weight, body proportions, and rate of growth. Neither pituitary weight nor brain weight appears to play the key role in determining which animals will exhibit growth deficiency.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of hypothalamic cell numbers in dwarf and normal weight rats exposed prenatally to methylazoxymethanol (MAM)

Growth deficiencies follow MAM exposure during the period when the growth hormone releasing factor (GRF) cells of the hypothalamus form, while animals exposed slightly later in gestation when the inhibitors of growth hormone release are forming, exhibit giantism. Counts of sample regions of the hypothalamus have shown that rats treated in utero on the 14th day of gestation have reductions in the number of GRF cells, increases in the number of SRIF (somatotropin release inhibiting factor) cells, and alterations of pituitary structure. These effects occurred in all treated subjects, even though obvious effects on body size were present in a small fraction of the treated animals. The present study was designed to examine the effect of 20 mg/kg MAM on the 13th day of gestation (a peak period for production of GRF cells) on GRF and SRIF cell numbers, in a large sample of dwarf-treated rats, normal weight-treated rats, and controls. The results of total counts of hypothalamic cells identified by immunocytochemistry demonstrated significant reductions in GRF cells in both dwarf and normal weight rats exposed to MAM, compared to controls, with no difference between the two treated groups. Like pituitary weights, the neuron counts were significantly correlated with body weight only in dwarf animals. SRIF cell numbers were equivalent to those in controls, suggesting that the increase reported earlier may have been due to a rebound effect in proliferation rather than some response of SRIF cells to GRF cell reduction.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of prenatal exposure to methylazoxymethanol (MAM) on brain weight, hypothalamic cell number, pituitary structure, and postnatal growth in the rat

Congenital brain damage syndromes typically are described in terms of behavioral symptoms. Many brain functions are not reflected in behavior, however, and prenatal injury to the developing nervous system could alter these functions, as well. To test the hypothesis that prenatal brain injury can result in postnatal endocrine malfunction, rats were exposed in utero to 20 mg/kg of methylazoxymethanol acetate, a potent neuroteratogen, at two stages of gestation when different sets of growth-controlling neurons of the hypothalamus are forming. The growth hormone releasing factor (GRF) neurons stimulate release of growth hormone from the somatotropes of the anterior pituitary, contributing to rapid growth in the period between weaning and puberty. The somatotropin release inhibiting factor (SRIF) neurons have the opposite effect on the pituitary and can inhibit the GRF cells directly. Growth of treated animals was monitored daily from birth to 40 days and compared to that of controls. Treatment on the 14th day of gestation produced a small number of dwarf animals characterized by normal weight at birth and a sudden decrease in growth rate at the beginning of the fourth postnatal week that led to a body weight about 50% of normal. Treatment on day 16 yielded an acceleration of postnatal growth (significant in males). In each group, most treated animals were like controls in adult size and pattern of growth. As adults, both treatment groups demonstrated massive reductions in brain weight which characterized all the subjects, whether or not they exhibited growth anomalies. The animals treated on day 14 were confirmed to have a significant, selective reduction in growth hormone releasing factor neurons. Reductions were greatest in the middle and posterior levels of the GRF cell distribution, the regions forming most actively at the time of exposure. Unexpectedly, the same group also had increased numbers of periventricular SRIF neurons. Neither type of neurons was significantly altered in the later treatment group. Examination of pituitary structure indicated that dwarfs had very small pituitaries, with an immature pattern of somatotrope distribution, and giants had very large pituitaries, with some hypertrophy of somatotropes. The results suggest that endocrine anomalies which manifest themselves long after birth may originate as birth defects of the nervous system.

Numbers of neurons and glia in mature rat somatosensory cortex: effects of prenatal exposure to ethanol

Stereological methods were used to examine the consequences of prenatal exposure to ethanol on the structure of area 3, primary somatosensory cortex, of the mature hooded rat. Pregnant rats were fed a liquid diet containing 6.7% (v/v) ethanol (Et), pair-fed an isocaloric liquid control diet (Ct), or fed a diet of chow and water (Ch). Cresyl violet-stained sections of 3-month-old pups were examined. The corrected mean size of the cell bodies of neurons in layers other than layer V was significantly smaller in the Et-treated rats; conversely, the mean somatic size of glia in each layer was significantly larger in the Et-treated rats. The laminar cell packing density for neurons and glia, however, was similar in rats from both treatment groups. The overall volume of area 3 and the volume of individual layers were about 33% smaller in Et-treated rats than in the pair-fed controls. Thus, the estimated total number of neurons in Et-treated rats (1.79 X 10(6] was significantly fewer than in Ch-treated rats (2.77 X 10(6] and in Ct-treated rats (2.66 X 10(6]. The total number of glia also was about 30% fewer in Et-treated rats than in the controls. Not all layers were affected equivalently. The space occupied by the neuropil was significantly greater in Et-treated rats, but only in layers II/III, IV, and VI; hence, the cell body/neuropil ratio in these layers was less in Et-treated rats than in the controls. Therefore, microcephaly caused by prenatal exposure to ethanol results not only from a miniaturization of the brain, but also from a permanent abnormal organization of cerebral cortex.

Birthdates of the growth hormone releasing factor cells of the rat hypothalamus: an autoradiographic study of immunocytochemically identified neurons

Growth hormone releasing factor (GRF) neurons in the arcuate nucleus of the hypothalamus and somatostatin (SRIF) neurons in the anterior periventricular region of the hypothalamus act to control the release of growth hormone from the anterior pituitary. To investigate the possibility that the growth-controlling functions of these cells might be compromised by injuries to the developing brain, it is important to know the details of the production and differentiation of these small, specialized cell groups. The overall pattern of cell production in the hypothalamus is known from autoradiographic studies with general nuclear stains, but no data are available on the birthdates (times of final mitoses) of GRF-producing cells. The present study was undertaken to determine when the GRF cells form. Counts of immunocytochemically identified GRF cells labeled on given days were taken from serial coronal sections through the hypothalamus of adult rats labeled on the 10th-17th days of gestation (day of finding a vaginal plug = day 1). As has been shown for the hypothalamus in general, the GRF cells showed a gradient of production from anterior to posterior. The peak of anterior cell proliferation was on day 13, middle cells on day 14, and posterior cells on day 15. These dates are 1 or 2 days earlier than those of GRF-negative cells in the same regions. No lateral to medial gradient of formation was seen in GRF cells. Rather, the laterally placed cells along the base of the brain and those surrounding the ventromedial nucleus formed simultaneously with the GRF cells of the arcuate nucleus. The birthdating results presented here are in agreement with the results of studies of teratogens which suggest that rat postnatal growth is reduced most severely by exposure to neurotoxic agents on days 12 or 13 of gestation. On the basis of data for the whole hypothalamus, such treatments would appear to be too early to interfere with cell production for the arcuate nucleus, but the timing fits the period of vulnerability as defined by the birthdates determined in the present study for the subpopulation of cells destined to produce GRF.

Morphometrical and microdensitometrical studies on peptide- and tyrosine hydroxylase-like immunoreactivities in the forebrain of rats prenatally exposed to methylazoxymethanol acetate

Methylazoxymethanol acetate (MAM Ac) injected into pregnant rats at a dose of 25 mg/kg at gestational day 15 causes microcephaly due to an atrophy of various telencephalic areas, mainly neocortex, hippocampus and basal ganglia. Previous studies demonstrated alterations in various neurochemical markers of classical transmitter systems in these regions. The present paper deals with changes in peptide and tyrosine hydroxylase (TH)-containing neurons in MAM Ac-induced microcephaly using immunocytochemistry coupled with computer-assisted morphometry and microdensitometry. No change in the number of vasoactive intestinal polypeptide (VIP)-immunoreactive neurons in the neocortex and neuropeptide Y (NPY)-immunoreactive neurons in the nucleus caudatus-putamen was found whereas cholecystokinin (CCK)-and NPY-immunoreactive neurons in the neocortex and CCK- and VIP-immunoreactive neurons in the hippocampus were decreased. The reduction of the latter peptide containing neuronal populations led to a maintained density of cells in MAM Ac-exposed rats, due to the parallel reduction of the overall mass of these regions. TH immunoreactivity was found to be unchanged in the basal ganglia, and increased in the cerebral cortex in agreement with previous reports on noradrenaline cortical system after MAM Ac exposure. The present results show a heterogenous vulnerability of different peptide immunoreactive neuronal populations to MAM Ac exposure. The sparing of VIP- and NPY-immunoreactive neurons may be due to their late development in the neocortex and striatum, respectively. The hypothesis is introduced that cortical VIP interneurons can develop independent of marked alterations in the intrinsic circuitry of the cortical region.

Hyperactivity and instrumental learning deficits in methylazoxymethanol-treated rat offspring

Several changes of spontaneous motor and learned behaviours were obtained in the male offspring of pregnant rats that were treated on gestation day 15 with the antimitotic agent methylazoxymethanol (MAM, 25 mg/kg). MAM-treated offspring, when tested at adult ages, showed notable increases in motor activity parameters as measured by direct observation or in automated photocell test cages. This hyperactive state was accompanied by clear impairments by MAM offspring in the acquisition of instrumental learning in a radial arm maze and in a circular swim maze. In Skinner boxes, MAM offspring made fewer responses during the Fixed Ratio (FR) 1 schedule but did not differ from the saline offspring in the acquisition of the difficult differential-reinforcement-of-low-rates (DRL) 72 sec task. Neurochemical assays indicated that the MAM rats had elevated noradrenaline and dopamine levels in several brain regions. These findings are discussed with regard to possible alterations of habituation processes in MAM rats.

Brain changes in rats induced by prenatal injection of methylazoxymethanol

Various doses (0, 1, 5, 10, 15, 20, or 25 mg/kg) of methylazoxymethanol acetate (MAM), a potent alkylating agent, were injected singly into pregnant rats intraperitoneally on day 15 of gestation. Relationships between brain weights and neurochemical changes in the cerebral hemispheres (CHs; cerebral cortex and subjacent white matter, hippocampus, amygdala) and remainder of the brain (BGDM; basal ganglia, diencephalon, and mesencephalon) were examined at 60 days of age in offspring; varying degrees of microencephaly were observed. Dose-dependent reductions in the weights of CH and BGDM were observed. Reductions in total DNA content positively correlated with decreases in brain weights also observed. Dose-dependent elevations of noradrenaline (NA) and dopamine (DA) were observed in CH at MAM levels 10 mg/kg and above; dose-dependent elevations of 5-hydroxytryptamine (5-HT) were observed at 15 mg/kg and above; and in BGDM at 20 mg/kg and above dose-dependent elevations for NA and 5-HT were observed; dose-dependent elevations at 15 mg/kg and above were observed for DA. Monoamine concentrations were negatively correlated with brain weights or total DNA contents. NA and DA concentrations increased to the extent of approximately 1.3 times of control at a time when an 18% loss of CH weight was noted in animals treated with 10 mg/kg MAM. It is suggested that the above variables might be appropriately sensitive neurochemical markers for detecting minor developmental anomalies in the brain.

The contribution of primary and secondary neuronal degeneration to prenatally-induced micrencephaly

Prenatal exposure of rats to the alkylating agent methylazoxymethanol acetate (MAM Ac) induces severe micrencephaly in the offspring. The aim of the present study was to examine the contribution of primary cell death (due to a direct action of MAM Ac on the neuroepithelium), and secondary (target-dependent) cell death to the subsequent cell deficits in the visual system following prenatal exposure to MAM Ac on embryonic day 11, 12, 13, 14, 15 or 16. The results showed that when primary cell death substantially reduced the neuronal population of a target structure then there was increased target-dependent cell death in the neurons which normally project to that target. This was particularly evident in the dorsal lateral geniculate nucleus following exposure to MAM Ac on E15. Although the MAM Ac caused virtually no primary cell death in the embryonic precursor cells of the dLGN, the nucleus in the adult offspring was reduced by 87% compared with controls. This reduction was shown to be due to increased postnatal target-dependent, or secondary, cell death due to a severe reduction in layers III and IV of the occipital cortex. The cortical damage was due to primary cell death. Hence, primary cell death only partly accounts for the degree of micrencephaly seen in the offspring, consideration of secondary cell death is necessary to understand the total deficit.

The development of daytime rearing behavior in methylazoxymethanol-treated rats: methodological considerations

The development of daytime rearing behavior was studied in the offspring of pregnant rats which received injections of methylaxymethanol acetate (MAM) or saline during the 15th day of gestation. MAM and control rats were tested at 10, 15, 20, 25, and 30 days of age. The results indicated that the onset of rearing for both groups appeared at approximately 15 days of age, with no significant differences found between sexes. No rearing deficits were seen in MAM rats through 25 days of age despite the fact that these animals sustained greater than a 50% reduction in telencephalic mass. However, at 30 days of age MAM rats reared for significantly longer periods of time during each episode than did their control counterparts, although the actual number of rears did not differ between groups. The results are discussed in terms of neuroplastic events which follow MAM-induced damage and the need for multivariate research when analyzing rearing behavior.

Impaired selective attention in methylazoxymethanol-induced microencephalic rats

Prenatal treatment of rats on gestation day 15 with methylazoxymethanol (MAM) caused forebrain microencephaly. Several behavioral tests were performed when the rats had reached an adult age. MAM treated rats were hyperactive, and were severely impaired in the acquisition of successive position reversal in a T-maze. The microencephalic rats failed also to demonstrate contextual control of latent inhibition (the stimulus preexposure effect) in taste-aversion conditioning. These results indicate that MAM treatment disrupts attentional processes and that this may account for the learning impairment.

Astroglial development in microencephalic rat brain after fetal methylazoxymethanol treatment

Treatment of pregnant rats on gestation day 15 with methylazoxymethanol (MAM) leads to a marked microencephaly in the offspring with a considerable atrophy in cerebral cortex, hippocampus and striatum. The development of the astrocytic populations in these atrophic regions was studied by means of immunohistochemistry using an antiserum against glial fibrillary acidic protein (GFA). The distribution and density of GFA-positive structures were not notably altered in the parietal cortex, hippocampal formation and striatum after prenatal MAM-treatment as compared to control. Also the individual astrocytes were morphologically similar in experimental and control animals in all regions analyzed. We suggest that an adjustment of the astrocytic development has occurred in response to the changed neuronal environment. Alternatively, MAM-treatment may affect neuronal and glial precursor cells leading to a seemingly normal astrocytic cell density.

The angiogenesis of micrencephalic rat brains caused by methylazoxymethanol acetate. I. Superficial venous system. A quantitative analysis

The angiogenesis of the rat cerebrum was studied under pathologic conditions caused by the administration of the neurotoxin methylazoxymethanol acetate (MAMAc) in the time (E14) of neuroblast migration. The sinovenous junction of the main superficial cerebral veins and the morphological changes of the veins were examined by a quantitative analytic method. The hypoplastic areas of the brains showed extremely malformed venous systems with pathologic changes of the sinovenous junctions depending on the degree of disturbance of the neuroblast migration. These findings suggest the primary role of the neuronal maturation in the angioarchitectonic development and the direct dependency of the vascular differentiation on the neuroblast migration of the drained territory.

Pervasive hyperactivity and long-term learning impairments in rats with induced micrencephaly from prenatal exposure to methylazoxymethanol

Pregnant Long-Evans rats were given a single i.p. injection of 30 mg/kg of methylazoxymethanol (MAM) acetate or saline on day 14 of gestation (vaginal plug = day 0). All litters were reduced to 8 at birth and were reared by their biological dams. Between 49-192 days of age all offspring were examined on open-field, figure-8 (at two different ages), and hole-board tests of activity, as well as passive avoidance and Biel water maze tests of learning (also at two different ages). The MAM offspring showed no increase in mortality, but weighed less than controls, a difference that remained relatively constant throughout the experiment. At 204-215 days of age the MAM offspring were confirmed to be micrencephalic, a known effect of this drug at this dose and exposure period. On all tests of activity the MAM offspring were markedly hyperactive. The female progeny also exhibited a pronounced impairment of normal activity habituation patterns. The MAM males, however, showed a marked impairment of passive avoidance performance, while the females did not. At 2 months of age the MAM offspring also showed a pronounced deficit in learning a water maze. This maze deficit had not abated when tested again at 6 months of age. The MAM induced brain and behavioral abnormalities provide a potentially useful animal model of congenital micrencephaly and associated mental retardation.

Infrapyramidal mossy fibers in the hippocampus of methylazoxymethanol acetate-induced microcephalic rats

Treatment of pregnant rats with methylazoxymethanol acetate results in the invasion of mossy fibers into the infrapyramidal region of the hippocampus in the offspring. Since such an invasion of mossy fibers has also been reported in neonatal hyperthyroidism, prenatal ethanol exposure and neonatal lesion of CA3, a common etiology for this phenomenon is proposed.

Effects of prenatal methylazoxymethanol treatment on the development of central monoamine neurons

The effects of prenatal treatment with the antimitotic agent methylazoxymethanol (MAM; 25 mg/kg i.v., gestation day 15) on the development of monoamine neurons and their receptors have been investigated by neurochemical techniques. The MAM treatment led to a forebrain microencephaly with an approximately 50% weight reduction of the cerebral cortex and hippocampus, 30% of the striatum while the other CNS regions were reduced by about 5-15%. Endogenous noradrenaline (NA) dopamine (DA) and 5-hydroxytryptamine (5-HT) concentrations in the cerebral cortex and hippocampus were increased by about 100% and to a similar extent for all amines, whereas the total amine content in each brain region analyzed was more or less unchanged after MAM treatment. The DA concentration in the striatum was increased by 40% without any change in the total DA content. The subcellular distribution of NA and DA in the cerebral cortex and striatum was similar in MAM-treated and control rats. The effect of the NA denervation agent DSP4 was identical in MAM-treated and controls, showing very pronounced NA reductions in the cerebral cortex, hippocampus, cerebellum and the spinal cord, while the DA levels in various brain regions were not or only to a minor degree affected. Analysis of [3H]NA and [3H]5-HT uptake in slices from the cerebral cortex in vitro, demonstrated an approximate doubling of the uptake (calculated per weight) for both amines after MAM, while total uptake was not notably changed. MAM treatment also led to a significant reduction of the in vitro binding of various radioligands for monoamine (alpha, beta, 5-HT) receptors. The most pronounced reduction was observed for beta-receptors. Administration of a tyrosine-hydroxylase (alpha-methyl-p-tyrosine) or a tryptophan hydroxylase (alpha-propyldopacetamide) inhibitor led to marked NA/DA and 5-HT reductions after MAM in all regions analyzed, indicating that the monoamine neurons are active in synthesizing and releasing their neurotransmitter. The results suggest that MAM treatment leads to a monoamine hyperinnervation in the atrophic regions without markedly changing the total number of monoamine nerve terminals. All the monoamine nerves appear to develop independent of the formation of the post-synaptic receptors. The results imply that monoamine neurons in the CNS are strictly programmed to produce a certain quantity of nerve terminal arborizations in regions they innervate during the development relatively independent of the effector cells, pointing to a high degree of intrinsic growth regulation.

Cytotoxic effects of ethylnitrosourea on central nervous system of rat embryos. Special references to carcinogenesis and teratogenesis

For the purpose of observing the cytotoxic effect of N-ethyl, N-nitrosourea (ENU), a potent carcinogen on the central nervous system (CNS) at the late period of organogenesis, the embryonal telencephalic wall was hourly examined under the light and electron microscopes. Pregnant rats on the 13th day and 15th day of gestation were given either a single intravenous injection of 40 or 80 mg/kg, of ENU. The cytotoxic effect on the embryo treated on the 15th day of gestation was severer than that treated on the 13th day of gestation. The common pathological changes in both treated groups are; (1) mitotic arrest 1 hour after administration in the most inner zone of the ventricular layer, (2) degeneration and necrosis accurred predominantly in the so-called DNA synthetic zone and resulting in cell loss of the ventricular layer, (3) elimination of degenerating products; and (4) tissue repair. The findings mentioned may indicate a cytotoxic effect inducing microencephalia, and furthermore, the teratogenic and carcinogenic mechanisms are discussed.

Sleep-waking cycle in microencephalic rats induced by prenatal methylazoxymethanol application

Sleep-wakefulness cycle of adult microencephalic rats due to prenatal administration of methylazoxymethanol (MAM) acetate at the dose of 20 mg/kg on the 15th day of gestation showed no difference from the normal control group, whereas at the dose of 25 mg/kg, %PS and PS/TS decreased significantly. MAM-induced microencephalic rats revealed a significant increase of spindle bursts during PS, suggesting that their PS is less deep than that of the controls.

Histological and neurochemical effects of fetal treatment with methylazoxymethanol on rat neocortex in adulthood

Forebrain microencephaly results when developing rats are exposed to methylazoxymethanol acetate (MAM) at 15 days of gestation (DG). This potent alkylating agent is selectively cytotoxic for dividing cells. Since distinct neuronal populations in neocortex vary greatly with respect to timing of mitotic activity during gestation, it was predicted that some groups would be differentially reduced by treatment. Histological examination of neocortex from treated rats grown to adulthood revealed major losses of laminae II--IV with relative preservation of deeper layers. The atrophic adult neocortex was further characterized by assay of several defined pre- and postsynaptic neurochemical markers. Total markers for GABAergic neurons were greatly reduced (glutamate decarboxylase -71%, [3H]GABA synaptosomal uptake -63% and endogenous GABA -59%). Total [3H]GABA binding to cortical membranes was reduced 67%. Total [3H]glutamate synaptosomal uptake and endogenous glutamate were reduced 71% and 65% respectively. In contrast, total presynaptic markers for noradrenergic innervation were minimally altered but concentration of tyrosine hydroxylase, [3H]norepinephrine synaptosomal uptake and endogenous norepinephrine were increased by 275%, 130% and 133%, respectively. Concentration of cholinergic presynaptic markers was also increased (choline acetyltransferase +97%, endogenous acetylcholine +64%) in atrophic cortex, but to a lesser degree than for noradrenergic innervation. Specific binding of muscarinic cholinergic antagonist [3H]quinuclidinyl benzilate and the beta-adrenergic receptor antagonist [3H]dihydroalprenolol was reduced 25% and 29% respectively in treated cortex. Thus, MAM treatment at 15 DG severely reduces intrinsic neuronal populations including GABAergic and glutamatergic neurons, and produces a shrunken cortex relatively hyperinnervated by noradrenergic and cholinergic neurons. MAM-induced microencephaly is a useful model system for producing relatively selective lesions of telencephalic neurons and for study of altered neurochemical relationships following developmentally incurred brain damage.

Methylazoxymethanol treatment of fetal rats results in abnormally dense noradrenergic innervation of neocortex

A single injection of methylazoxymethanol in pregnant rats at 15 days of gestation results in severe cortical atrophy in the offspring. In the adult offspring, the neurochemical markers for the cortical gamma-aminobutyric acid-containing neurons are severely reduced, whereas the noradrenergic markers are minimally altered. Immunohistofluorescence microscopy demonstrates a marked increase in the density of noradrenergic axons which have an abnormal pattern of distribution in the atrophic cortex. The results suggest that the central noradrenergic neurons determine the number of axons to be formed early in brain development, but local factors in the terminal field regulate the ultimate distribution of the noradrenergic axons.

Methylazoxymethanol acetate: effect of postnatal injection on brain amines and behavior

The antimitotic drug, methylazoxymethanol acetate (MAMA), was injected into newborn rats during the first four days of life. At 48 days of age, these rats weighed one-third less than controls, as did the cerebella of their brains, but the rest of their brains weighed only 7% less than those of controls. The cerebella structures of the drug-injected rats was highly disorganized. Purkinje cells were scattered haphazardly in the granular layer instead of forming a monolayer. More foldings and short folia were found in the cerebella of drugged animals. In spite of these large morphological differences, the total amounts of norepinephrine and serotonin in the cerebella of the drugged rats were not different from those of the control rats. Behavioral effects of postnatal injection of MAMA include retarded development of the righting reflex,i.e., the drugged pups took longer time to right themselves when placed on their backs during the first nine days after birth; and scondly, MAMA reduced locomotor activity measured 45 days after birth.

Ectopic neurones in the hippocampus of the postnatal rat exposed to methylazoxymethanol during foetal development

Ectopic neurons have been detected in the hippocampus of postnatal hooded rats aged 5-24 days. These rats were exposed to methylazoxymethanol acetate (MAMac) during foetal development by injecting the mother rats with this neurotoxin. At birth, the hippocampus of rats exposed to MAMac showed a normal cytoarchitecture; ectopic neurones became prevalent from 5 days onwards. These ectopic neurones were restricted to subfields CA 1-2 of Ammon's horn, and evidence is presented which suggests that these ectopic sites are formed by neuronal emigration from stratum pyramidale. Ectopic neurones have been shown to occur in genetic abnormalities of man, and in the mutant mouse 'reeler'. MAMac is a powerful methylating agent especially for guanine which is present in DNA and RNA. It is postulated that viable cells with an altered DNA-RNA state may in some way be predisposed to the formation of ectopic cell clusters after a latent period. The movement of neurones from stratum pyramidale provides a convenient animal model for investigating the mechanisms by which ectopic neuronal sites are formed by abnormal migratory patterns.

Short and long term effects of methyl- and ethylnitrosourea (MNU & ENU) on the developing nervous system of the rat. II. Short term effects: concluding remarks on chemical neuro-oncogenesis

The cytotoxic action of various single doses of MNU and ENU on developing neural and extraneural tissues was studied at different stages of development. Examination revealed lethal damage. (L.I.) and mitotic inhibition (M.I.), confined to proliferating cells only, and caused by the number of alkyl groups administered. In studying the duration of M.I. a difference was found in duration of the cell cycle arrest after MNU or ENU. The arrest lasted longer for MNU than for ENU, and the neural tissues turned out to be more sensitive than the extraneural ones. Moreover, among the reappearing mitotic figures abnormal ones were noticed frequently. After pulse-labeling with thymidine this arrest could be traced to take place in or before entering the S-phase. During the period of this arrest a low, but specific, activity was found that might point to the existence of repair-processes in vivo. Finally, we directly demonstrated alkylations in tissue-sections by the use of (14C-methyl)-MNU. High radioactivity was found with a random distribution over the various tissues, cell types and even cellular compartments. Therefore--in contrast with the cytotoxic effects--alkylation seems to occur in all cell types. In conclusion, it seems justified to consider the matrices of proliferating cells in the central nervous system as the target tissue-areas for the carcinogenic action of both MNU and ENU. Re-entrance of these damaged cells into their cycle prior to the elimination of altered bases from DNA might be of great importance for the problem of oncogenesis.