Selective ablation of neurons by methylazoxymethanol during pre- and postnatal brain development

Genotoxic Damage During Brain Development Presages Prototypical Neurodegenerative Disease

Western Pacific Amyotrophic Lateral Sclerosis and Parkinsonism-Dementia Complex (ALS/PDC) is a disappearing prototypical neurodegenerative disorder (tau-dominated polyproteinopathy) linked with prior exposure to phytogenotoxins in cycad seed used for medicine and/or food. The principal cycad genotoxin, methylazoxymethanol (MAM), forms reactive carbon-centered ions that alkylate nucleic acids in fetal rodent brain and, depending on the timing of systemic administration, induces persistent developmental abnormalities of the cortex, hippocampus, cerebellum, and retina. Whereas administration of MAM prenatally or postnatally can produce animal models of epilepsy, schizophrenia or ataxia, administration to adult animals produces little effect on brain structure or function. The neurotoxic effects of MAM administered to rats during cortical brain development (specifically, gestation day 17) are used to model the histological, neurophysiological and behavioral deficits of human schizophrenia, a condition that may precede or follow clinical onset of motor neuron disease in subjects with sporadic ALS and ALS/PDC. While studies of migrants to and from communities impacted by ALS/PDC indicate the degenerative brain disorder may be acquired in juvenile and adult life, a proportion of indigenous cases shows neurodevelopmental aberrations in the cerebellum and retina consistent with MAM exposure in utero. MAM induces specific patterns of DNA damage and repair that associate with increased tau expression in primary rat neuronal cultures and with brain transcriptional changes that parallel those associated with human ALS and Alzheimer's disease. We examine MAM in relation to neurodevelopment, epigenetic modification, DNA damage/replicative stress, genomic instability, somatic mutation, cell-cycle reentry and cellular senescence. Since the majority of neurodegenerative disease lacks a solely inherited genetic basis, research is needed to explore the hypothesis that early-life exposure to genotoxic agents may trigger or promote molecular events that culminate in neurodegeneration.

Western Pacific ALS-PDC: Evidence implicating cycad genotoxins

Amyotrophic Lateral Sclerosis and Parkinsonism-Dementia Complex (ALS-PDC) is a disappearing neurodegenerative disorder of apparent environmental origin formerly hyperendemic among Chamorros of Guam-USA, Japanese residents of the Kii Peninsula, Honshu Island, Japan and Auyu-Jakai linguistic groups of Papua-Indonesia on the island of New Guinea. The most plausible etiology is exposure to genotoxins in seed of neurotoxic cycad plants formerly used for food and/or medicine. Primary suspicion falls on methylazoxymethanol (MAM), the aglycone of cycasin and on the non-protein amino acid β-N-methylamino-L-alanine, both of which are metabolized to formaldehyde. Human and animal studies suggest: (a) exposures occurred early in life and sometimes during late fetal brain development, (b) clinical expression of neurodegenerative disease appeared years or decades later, and (c) pathological changes in various tissues indicate the disease was not confined to the CNS. Experimental evidence points to toxic molecular mechanisms involving DNA damage, epigenetic changes, transcriptional mutagenesis, neuronal cell-cycle reactivation and perturbation of the ubiquitin-proteasome system that led to polyproteinopathy and culminated in neuronal degeneration. Lessons learned from research on ALS-PDC include: (a) familial disease may reflect common toxic exposures across generations, (b) primary disease prevention follows cessation of exposure to culpable environmental triggers; and (c) disease latency provides a prolonged period during which to intervene therapeutically. Exposure to genotoxic chemicals ("slow toxins") in the early stages of life should be considered in the search for the etiology of ALS-PDC-related neurodegenerative disorders, including sporadic forms of ALS, progressive supranuclear palsy and Alzheimer's disease.

Early cognitive experience prevents adult deficits in a neurodevelopmental schizophrenia model

Brain abnormalities acquired early in life may cause schizophrenia, characterized by adulthood onset of psychosis, affective flattening, and cognitive impairments. Cognitive symptoms, like impaired cognitive control, are now recognized to be important treatment targets but cognition-promoting treatments are ineffective. We hypothesized that cognitive training during the adolescent period of neuroplastic development can tune compromised neural circuits to develop in the service of adult cognition and attenuate schizophrenia-related cognitive impairments that manifest in adulthood. We report, using neonatal ventral hippocampus lesion rats (NVHL), an established neurodevelopmental model of schizophrenia, that adolescent cognitive training prevented the adult cognitive control impairment in NVHL rats. The early intervention also normalized brain function, enhancing cognition-associated synchrony of neural oscillations between the hippocampi, a measure of brain function that indexed cognitive ability. Adolescence appears to be a critical window during which prophylactic cognitive therapy may benefit people at risk of schizophrenia.

A distinctive layering pattern of mouse dentate granule cells is generated by developmental and adult neurogenesis

New neurons are continuously added throughout life to the dentate gyrus of the mammalian hippocampus. During embryonic and early postnatal development, the dentate gyrus is formed in an outside-in layering pattern that may extend through adulthood. In this work, we sought to quantify systematically the relative position of dentate granule cells generated at different ages. We used 5'-bromo-2'-deoxyuridine (BrdU) and retroviral methodologies to birth date cells born in the embryonic, early postnatal, and adult hippocampus and assessed their final position in the adult mouse granule cell layer. We also quantified both developmental and adult-born cohorts of neural progenitor cells that contribute to the pool of adult progenitor cells. Our data confirm that the outside-in layering of the dentate gyrus continues through adulthood and that early-born cells constitute most of the adult dentate gyrus. We also found that substantial numbers of the dividing cells in the adult dentate gyrus were derived from early-dividing cells and retained BrdU, suggesting that a subpopulation of hippocampal progenitors divides infrequently from early development onward.

Neurodegeneration in schizophrenia

The neurodegenerative aspect of schizophrenia presupposes gene-environmental interactions involving chromosomal abnormalities and obstetric/perinatal complications that culminate in predispositions that impart a particular vulnerability for drastic and unpredictable precipitating factors, such as stress or chemical agents. The notion of a neurodevelopmental progression to the disease state implies that early developmental insults, with neurodegenerative proclivities, evolve into structural brain abnormalities involving specific regional circuits and neurohumoral agents. This neurophysiological orchestration is expressed in the dysfunctionality observed in premorbid signs and symptoms arising in the eventual diagnosis, as well as the neurobehavioral deficits reported from animal models of the disorder. The relative contributions of perinatal insults, neonatal ventral hippocampus lesion, prenatal methylazoxymethanol acetate and early traumatic experience, as well as epigenetic contributions, are discussed from a neurodegenerative view of the essential neuropathology. It is implied that these considerations of factors that exert disruptive influences upon brain development, or normal aging, operationalize the central hub of developmental neuropathology around which the disease process may gain momentum. Nonetheless, the status of neurodegeneration in schizophrenia is somewhat tenuous and it is possible that brain imaging studies on animal models of the disorder, which may describe progressive alterations to cortical, limbic and ventricular structures similar to those of schizophrenic patients, are necessary to resolve the issue.

Gestational methylazoxymethanol acetate treatment impairs select cognitive functions: parallels to schizophrenia

Gestational methylazoxymethanol acetate (MAM) exposure has been suggested to produce neural and behavioral abnormalities similar to those seen in schizophrenia. In order to assess MAM treatment as a model of schizophrenia, pregnant female rats were injected with MAM (22 mg/kg) on gestational day 17 and their offspring were assessed in adulthood on a series of cognitive tasks. The first experiment involved an attentional set-shifting task, a rodent analog of the Wisconsin card sort task. In experiment 2, animals were tested on the 5-choice serial reaction time task, a rodent analog of the continuous performance task. In the final experiment animals were assessed on a differential reinforcement of low rate of responding 20 s schedule of reinforcement (DRL-20), a task that is sensitive to changes in inhibitory control. In the first experiment, MAM-treated animals required a greater number of trials than controls to successfully learn an extradimensional shift on the set-shifting task, and had difficulties in learning to reverse a previously acquired discrimination. In contrast, MAM-treated animals showed little impairment on the 5-choice task, aside from a modest but consistent increase in premature responding. Finally, MAM exposed animals showed substantial impairments in DRL performance. Post-mortem analysis of brain tissue showed significant decreases in tissue weight in the hippocampus, parietal cortex, prefrontal cortex, and dorsal striatum of MAM-treated animals. These results support the notion that MAM treatment may simulate some aspects of schizophrenic cognition.

Pilocarpine-induced seizure susceptibility in rats following prenatal methylazoxymethanol treatment

Several rodent models of cortical malformation are available for the study of cellular mechanisms associated with early-onset epilepsy, but few are associated with spontaneous seizures. We examined the effect of pilocarpine on the spontaneous seizure development and excitability of the CA1 pyramidal cells of rats after prenatal treatment with methylazoxymethanol (MAM). Pilocarpine induced status epilepticus (SE) onset latency was greater for normal rats than for MAM-treated rats. After several days of normal behavior following pilocarpine treatment, the duration of spontaneous seizures were greater in MAM-pilocarpine rats than in normal-pilocarpine rats. Compared with the normal rats, electrical stimulation of afferent fibers resulted in more robust population responses in the CA1 region in all groups. At interstimulus intervals of 30 and 70 ms, the MAM-pilocarpine rats displayed a decrease in paired pulse inhibition versus the conventional MAM rats. A loss of somatostatin- and parvalbumin-immunoreactive neurons was apparent in the normal-pilocarpine rats, MAM-pilocarpine rats, and conventional MAM rats. These results indicate that pilocarpine induces spontaneous seizures and hyperexcitability in MAM-pilocarpine rats.

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.

Analysis of cerebellar development in math1 null embryos and chimeras

The cerebellar granule cell is the most numerous neuron in the nervous system and likely the source of the most common childhood brain tumor, medulloblastoma. The earliest known gene to be expressed in the development of these cells is math1. In the math1 null mouse, neuroblasts never populate the external germinal layer (EGL) that gives rise to granule cells. In this study, we examined the embryonic development of the math1 null cerebellum and analyzed experimental mouse chimeras made from math1 null embryos. We find that the anterior rhombic lip gives rise to more than one cell type, indicating that the rhombic lip does not consist of a homogeneous population of cells. Furthermore, we demonstrate that math1 null granule cells are absent in the math1 null chimeric cerebellum, from the onset of their genesis in the mouse anterior rhombic lip. This finding indicates a vital cell intrinsic role for Math1 in the granule cell lineage. In addition, we show that wild-type cells are unable to compensate for the loss of mutant cells. Finally, the colonization of the EGL by wild-type cells and the presence of acellular gaps provides evidence that EGL neuroblasts undergo active migration and likely have a predetermined spatial address in the rhombic lip.

Experimentally-induced microencephaly: effects on cortical neurons

Genetic and epigenetic factors may alter the normal development of cerebral cortex, producing laminar and cellular abnormalities and heterotopiae, major causes of juvenile, drug-resistant epilepsy. Experimentally-induced migration disorders provide interesting insights in the mechanisms of the determination of neuronal phenotype and connectivity, of congenital cortical dysgenesis and the pathophysiology of associated neurological disorders, such as epilepsy. We investigated the effects of E14 administration of methylazoxymethanol acetate (MAM), which induces microencephaly by ablating dividing cells. Brains from newborn and adult rats were reacted for NADPH-d and CO histochemistry. Moreover, callosally-projecting neurons were retrogradely labeled with DiI at P9 or with BDA in adults. MAM-treated rats displayed a remarkable reduction in cortical thickness, mainly due to reduction in layer IV and in supragranular layers. Heterotopic nodules appeared in the supragranular layers and in the hippocampus. CO-positive barrels in somatosensory cortex were almost absent. The distribution of NADPH-d-positive neurons was regular, but they were rare in heterotopic nodules. Callosally-projecting neurons displayed abnormal orientation of the apical dendrite and increase in the basal dendritic length. Alterations in the dendritic arborization of pyramidal neurons may be one of the substrates for the increased sensitivity to drugs which induce epileptic seizures in these animals.

NPY sensitivity and postsynaptic properties of heterotopic neurons in the MAM model of malformation-associated epilepsy

Neuronal migration disorders (NMDs) can be associated with neurological dysfunction such as mental retardation, and clusters of disorganized cells (heterotopias) often act as seizure foci in medically intractable partial epilepsies. Methylazoxymethanol (MAM) treatment of pregnant rats results in neuronal heterotopias in offspring, especially in hippocampal area CA1. Although the neurons in dysplastic areas in this model are frequently hyperexcitable, the precise mechanisms controlling excitability remain unclear. Here, we used IR-DIC videomicroscopy and whole cell voltage-clamp techniques to test whether the potent anti-excitatory actions of neuropeptide Y (NPY) affected synaptic excitation of heterotopic neurons. We also compared several synaptic and intrinsic properties of heterotopic, layer 2-3 cortical, and CA1 pyramidal neurons, to further characterize heterotopic cells. NPY powerfully inhibited synaptic excitation onto normal and normotopic CA1 cells but was nearly ineffective on responses evoked in heterotopic cells from stimulation sites within the heterotopia. Glutamatergic synaptic responses on heterotopic cells exhibited a comparatively small, D-2-amino-5-phosphopentanoic acid-sensitive, N-methyl-D-aspartate component. Heterotopic neurons also differed from normal CA1 cells in postsynaptic membrane currents, possessing a prominent inwardly rectifying K(+) current sensitive to Cs(+) and Ba(2+), similar to neocortical layer 2-3 pyramidal cells. CA1 cells instead had a prominent Cs(+)- and 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride-sensitive I(h) and negligible inward rectification, unlike heterotopic cells. Thus heterotopic CA1 cells appear to share numerous physiological similarities with neocortical neurons. The lack of NPY's effects on intra-heterotopic inputs, the small contribution of I(h), and abnormal glutamate receptor function, may all contribute to the lowered threshold for epileptiform activity observed in hippocampal heterotopias and could be important factors in epilepsies associated with NMDs.

Dissection of the cellular and molecular events that position cerebellar Purkinje cells: a study of the math1 null-mutant mouse

Granule cell precursors in the external germinal layer (EGL) of the cerebellum have been proposed to be a major player in the migration and positioning of Purkinje cells through the expression of the Netrin-like receptor Unc5h3 and the extracellular matrix molecule Reelin. To explore the role of the EGL on these processes, we made use of the math1 null-mutant mouse in which the EGL does not form. In the absence of the EGL, we find three populations of ectopic Purkinje cells. First, we find 1% of all Purkinje cells in a supracerebellar position at the dorsal midline. Second, we find 7% of all Purkinje cells in the inferior colliculus, similar to what is seen in the Unc5h3 mutant. Our finding that Unc5h3 expression is not disrupted in these cells supports the proposed role of EGL granule cell precursors in establishing the anterior cerebellar boundary through the expression of Unc5h3. Third, we find 20% of all Purkinje cells positioned deep to the cerebellar cortex as seen in the reeler mutant. However, unlike the reeler mutant, where 5% of the Purkinje cells migrate successfully, we find that in the math1 null that 72% of the Purkinje cells migrate successfully. This finding demonstrates that Purkinje cell migration is not solely dependent on Reelin signaling from the EGL and is likely caused by Reelin signals emanating from the nuclear transitory zone or the ventricular zone, or both.

Postnatal development of rat dentate gyrus: effects of methylazoxymethanol administration

In order to investigate the role of postnatal neurogenesis in granule cell number control in the rat dentate gyrus, we administered Methylazoxymethanol (MAM), a drug able to prevent cells from dividing, on P3, P5, P7, P9, when the most granule cells are produced. The effect of MAM on the number of proliferating precursors and of granule cells was examined at P16 and P90. We used 5-bromo-2'-deoxyuridine administration to label proliferating cells and immunohistochemistry to characterize the cell phenotype using neuron markers TUC 4, PSA-NCAM, Calbindin D28K and glial marker GFAP. At 16 days of age in MAM-treated rats we observed a significant decrease of BrdU-positive cells. Consistently, a decrease in density and number of granule cells was found compared to the controls. At 90 days the dentate gyrus of treated rats showed a complete recovery: no differences in the density, total number of neurons, the BrdU- and TUC 4-positive cells were revealed with respect to the controls. No deficits were evident in performance on the water maze in MAM-treated rats. These data suggest that the dentate gyrus is able to re-establish the proliferative zone and to rebuild the granule cell layer following neonatal MAM administration.

Neuronal microdysgenesis and acquired lesions of the hippocampal formation connected with seizure activities in Ihara epileptic rat

The present study was designed to examine the morphological features of the hippocampal formation in the Ihara epileptic rat (IER), and to characterize genetically programmed lesions and acquired lesions connected with seizure activities. Neuropathological investigation of the hippocampal formation was performed in four separate groups, 2-month-old IERs with neither abnormal behaviors nor any seizure activity, and 12-month-old IERs of both sexes with abnormal behaviors, circling seizures or generalized tonic-clonic convulsions. In every IER examined, there were invariable and fundamental neuropathological findings consisting of abnormal neuronal clusters in the CA1 of the hippocampal formation. Moreover, disarrangement of neuronal cells, such as dispersion and gaps in lamination of pyramidal neurons, were observed. These changes were thought to represent genetically programmed lesions, neuronal microdysgenesis, because they were common findings in 2-month-old and 12-month-old IERs of both sexes. An enlargement of the dentate gyrus was also found in rats that experienced generalized tonic-clonic convulsions or circling seizures. This enlargement of the dentate gyrus, on the other hand, was categorized as a secondary and acquired lesion connected with seizure activities. It is suggested that the neuronal microdysgenesis in the hippocampal formation of IER has an intimate relationship with epileptogenesis and/or an enhancement of seizure susceptibility.

Mechanisms underlying epileptogenesis in cortical malformations

The presence of developmental cortical malformations is associated with epileptogenesis and other neurological disorders. In recent years, animal models specific to certain malformations have been developed to study the underlying epileptogenic mechanisms. Teratogens (chemical, thermal or radiation) applied during cortical neuroblast division and migration result in lissencephaly and focal cortical dysplasia. Animals with these malformations have a lowered seizure threshold as well as histopathologies typical of those found in human dysgenic brains. Alterations that may promote epileptogenesis have been identified in lissencephalic brains, such as increased numbers of bursting types of neurons, and abnormal connections between hippocampus, subcortical heterotopia, and neocortex. A distinct set of pathological properties is present in animal models of 4-layered microgyria, induced with cortical lesions made during late stages of cortical neuroblast migration. Hyperexcitability has been demonstrated in cortex adjacent to the microgyrus (paramicrogyral zone) in in vitro slice preparations. A number of observations suggest that cellular differentiation is delayed in microgyric brains. Other studies show increases in postsynaptic glutamate receptors and decreases in GABA(A) receptors in microgyric cortex. These alterations could promote epileptogenesis, depending on which cell types have the altered receptors. The microgyrus lacks thalamic afferents from sensory relay nuclei, that instead appear to project to the paramicrogyral region, thereby increasing excitatory connectivity within this epileptogenic zone. These studies have provided a necessary first step in understanding molecular and cellular mechanisms of epileptogenesis associated with cortical malformations.

Neurotoxic effects of phenytoin on postnatal mouse brain development following neonatal administration

Phenytoin (PHT) is a commonly used anticonvulsant drug. It has been reported that children exposed prenatally to PHT have brain malformations and psychomotor dysfunction. The neonatal development of the central nervous system (CNS) in mice corresponds to the last trimester in humans. To examine the neurotoxic effects of PHT on postnatal brain development, we administered PHT at doses of 10, 17.5, 25, or 35 mg/kg to newborn mice once a day during postnatal days (PD) 2-4. These dose levels result in plasma levels corresponding to the therapeutic ranges in humans. We measured the weight of total brain, cerebrum, cerebellum, and brain stem on PD 5 through 21, and examined early motor functions including head elevation, elevation of pelvis, pivoting, crawling, and righting reflex . Total brain weight, cerebral weight, and cerebellar weight in the group treated with 25 or 35 mg/kg were significantly reduced compared to controls from PD 5 to 21. Mice treated with PHT at 25 or 35 mg/kg showed decreased locomotor abilities and righting reflex on PD 5. In all phenytoin treatment groups, phenytoin levels in the brain were higher than those in the plasma on the third day of PHT treatment. We thus observed neurotoxic effects of PHT on postnatal brain development in mice. Our present data may provide useful implications for the management of PHT-induced developmental neurotoxicity and evaluation of psychomotor development in children exposed to PHT during the late fetal period.

Alteration of neuronal nitric oxide synthase activity and expression in the cerebellum and the forebrain of microencephalic rats

Microencephalic rats were obtained through gestational (for the forebrain) or neonatal (for the cerebellum) administration of the DNA-alkylating agent methylazoxymethanol acetate (MAM), which selectively kills dividing cells during neurogenesis. In the microencephalic cerebellum the specific activity of calcium-dependent nitric oxide synthase (NOS) was decreased by 35-40% at 12, 28 and 70 days of age. Other neurochemical markers not related to granule cells (the neuronal population selectively compromised by neonatal MAM treatment), choline acetyltransferase (ChAT) and glutamate decarboxylase (GAD) were not decreased, but actually increased when determined as specific activity. In agreement with the decreased catalytic activity measured in the tube, the expression of neuronal NOS protein was attenuated as judged from immunohistochemistry and Western blotting. In the microencephalic forebrain, the specific calcium-dependent NOS activity measured in homogenates of the whole hemisphere was significantly increased as compared to normal animals. Accordingly, immunohistochemistry for neuronal NOS, as well as NADPH-diaphorase histochemistry revealed an apparent increase in the density of strongly reactive neurons in the underdeveloped cortex and striatum of microencephalic rats. The results reported here demonstrate that permanent alterations of neuronal NOS activity and expression occur when the development of the brain and its neuronal circuits are severely compromised. Furthermore, the permanent downregulation of neuronal NOS in the cerebellum of microencephalic rats may be exploited for the study of the role of NO in mechanisms of synaptic plasticity such as long term depression (LTD).

Neocortex in the hippocampus: an anatomical and functional study of CA1 heterotopias after prenatal treatment with methylazoxymethanol in rats

Migration disorders cause neurons to differentiate in an abnormal heterotopic position. Although significant insights have been gained into the etiology of these disorders, very little is known about the anatomy of heterotopias. We have studied heterotopic masses arising in the hippocampal CA1 region after prenatal treatment with methylazoxymethanol (MAM) in rats. Heterotopic cells were phenotypically similar to neocortical supragranular neurons and exhibited the same temporal profile of migration and neurogenesis. However, they did not express molecules characteristic of CA1 neurons such as the limbic-associated membrane protein. Horseradish peroxidase injections in heterotopia demonstrated labeled fibers not only in the neocortex and white matter but also in the CA1 stratum radiatum and stratum lacunosum. To study the pathophysiological consequences of this connectivity, we compared the effects of neocortical and limbic seizures on the expression of Fos protein and on cell death in MAM animals. After metrazol-induced seizures, Fos-positive cells were present in CA1 heterotopias, the only hippocampal region to be activated with the neocortex. By contrast, kainic acid-induced seizures caused a prominent delayed cell death in limbic regions and in CA1 heterotopias. Together, these results suggest that neocortical heterotopias in the CA1 region are integrated in both the hippocampal and neocortical circuitry.

Distribution of metabotropic glutamate receptor type 1a in Purkinje cell dendritic spines is independent of the presence of presynaptic parallel fibers

The metabotropic glutamate receptor type 1a (mGluR1a) is expressed at a high level in the molecular layer of the cerebellar cortex, where it is localized mostly in dendritic spines of Purkinje cells, innervated by parallel fibers. Treatment with methylazoxymethanol (MAM) of mouse pups at postnatal days (PND) 0 + 1 or 5 + 6 results in the partial loss of granule cells, the extent of which depends on the age of the animal at the time of injection. As a consequence of hypogranularity, the number of parallel fibers is decreased to such an amount that many of the postsynaptic Purkinje cell dendritic spines are devoid of axonal input, and only a limited number of spines participate in the formation of parallel fiber synapses, or, infrequently, in heterologous or heterotopic synapses with other presynaptic partners. At PND 30, 50% of the spines in the cerebella of mice treated with MAM at PND 0 + 1 was not contacted by any presynaptic element, compared to 5% in controls or 15% in the cerebella of mice treated with MAM at PND 5 + 6. The localization of mGluR1a was visualized by immunocytochemistry on ultrathin sections: approximately 80% of all Purkinje cell dendritic spines were immunopositive in controls and in both groups of MAM-treated mice, indicating that mGluR1a was present in Purkinje dendritic spines even when the corresponding synaptic input was absent. This observation indicates that the expression and subcellular distribution of mGluR1a are inherent, genetically determined properties of Purkinje cells.

Ischemic and excitotoxic damage to brain slices from normal and microencephalic rats

Brain slices (olfactory cortex, fronto-parietal cortex and hippocampus) taken from normal or microencephalic rats, obtained by gestational administration of the DNA-alkylating agent methylazoxymethanol acetate (MAM), were subjected to in vitro simulated ischemia or exposed to glutamate (5 mM) or kainate (1 mM). All these neurotoxic insults resulted in decreased viability of the slices, as quantitatively assessed by decrease in the rate of protein synthesis. Hippocampal slices subjected to ischemia and olfactory cortex slices exposed to glutamate or kainate were significantly less sensitive to the neurotoxic insult in microencephalic rats than in controls. The increased efflux of neurotransmitter amino acids (glutamate, aspartate and GABA) in the medium from slices subjected to ischemia or exposed to kainate, showed no significant differences among microencephalic and control rats. The present results suggest that the decreased excitotoxic sensitivity of microencephalic rats is, at least in part, related to intrinsic structural and/or functional alterations of some brain regions which undergo decrease in size as a consequence of the gestational treatment.

Neuroanatomical and functional alterations resulting from early postnatal cerebellar insults in rodents

This review examines neuroanatomical and functional alterations in rodents resulting from postnatal insults during cerebellar development. Treatments such as irradiation and methylazoxymethanol (MAM) administration produced near birth (< postnatal day 8 for irradiation treatment and < postnatal day 4 for MAM administration) result in more severe cerebellar damage than do similar treatments administered several days after birth. Prominent among the more severe alterations are foliation abnormalities, misalignment of Purkinje cells and continued multiple innervation of climbing fibers; few or none of these occur as a result of later treatments (> postnatal day 8 for irradiation treatment and > postnatal day 4 for MAM treatment). The functional alterations also differ: insults produced near birth result in hypoactivity, ataxia, tremor and accompanying learning deficits, whereas those produced later result in hyperactivity and few learning deficits. This hyperactivity may have relevance to human disorders. Brief discussions of cerebellar and functional alterations (e.g., hyperactivity) resulting from neonatal infection with the Borna disease virus and induction of hypo- and hyperthyroidism during the preweaning period are also presented.

Short exposure to methylazoxymethanol causes a long-term inhibition of axonal outgrowth from cultured embryonic rat hippocampal neurons

Methylazoxymethanol (MAM) is an alkylating agent that is used to induce microencephaly by killing mitotically active neuroblasts. We found that at later developmental times, MAM exposure can result in abnormal fiber growth in vivo. However, there have not been any previous studies on the effects of MAM on differentiating neurons. We examined the outcome of short exposure to MAM on postmitotic embryonic hippocampal cultures during the establishment of axonal polarity. At 0, 1, or 2 days in vitro (DIV), neurons were treated with 0.1 nM-1 microM MAM for 3 hr and then transferred to glial conditioned media. At 3 DIV, the cells were fixed and analyzed by immunofluorescent staining for neuron viability and differentiation. Control cells initiate several minor processes; one process elongates rapidly at about 1 DIV eventually becoming an axon, while extensive dendritic growth occurs after 3-4 DIV. Neurons treated with 1 microM MAM at 0 or 1 DIV showed a marked inhibition of neurite growth and withdrawal of axons without affecting cell viability. These cells continued to show minimal neurite outgrowth at 7 DIV, even when transferred to a glial coculture. In contrast, cells treated initially with MAM, after neuronal polarity is established at 2 DIV, showed no effect on axonal growth. To determine the effects of MAM on the neuronal cytoskeleton, we examined the in vitro assembly of brain microtubules in a one cycle assay. Exposure to MAM depleted the soluble pool of proteins, including microtubule-associated protein 1B (MAP1B) and MAP2, which are required for neurite outgrowth, through a nonspecific process. Under non-saturating conditions, there were no changes in the total amount of microtubules assembled or the coassembly of MAP1B and MAP2 in the presence of MAM. These results demonstrate that MAM can directly affect differentiating neurons, indicating that an early disruption of axonal outgrowth may have long-term effects.

Absence of excitotoxic neuropathology in microencephalic rats after systemic kainic acid administration

We have s.c. injected with kainic acid (12 mg/kg) normal adult rats as well as rats rendered microencephalic by selectively timed administration of the DNA alkylating agent methylazoxymethanol acetate (MAM) to the mother during pregnancy. Histological examination of the brains revealed that normal animals underwent neurodegeneration in brain regions sensitive to kainic acid excitotoxicity, such as the olfactory cortex and the hippocampus, while no damage was apparent in the same regions of microencephalic rats. Evaluation of the neurotoxic outcome consequent to the excitotoxic stimulation, was quantitatively performed by measuring the levels of appropriate neurochemical markers 15 days after kainic acid injection. In normal animals, this resulted in significant decrease (up to 60% in the olfactory cortex and 30% in the hippocampus) of markers related to glutamatergic and GABAergic neurons, whereas in MAM-treated rats the same markers were not significantly affected, thus demonstrating a substantial protection against the excitotoxic insult in the microencephalic condition.

Functional effects of methylazoxymethanol-induced cerebellar hypoplasia in rats

The behavioral effects of a series of methylazoxymethanol acetate (MAM) injections in neonatal rats were investigated. Pups were injected twice daily on days 5-8 after birth with 4 mg/kg MAM or saline. Similar treatment paradigms cause cerebellar hypoplasia, which is a result of a depletion of granule cells. MAM treatment reduced adult cerebellar weight to 92% that of control and was without effect on weight of other brain regions examined. Postweaning body weight gain in males was reduced. Nest odor preference and emergence (light/dark box) assessments indicated no significant effects. Complex maze assessments and performance in an operant test battery demonstrated no cognitive deficits. Indeed, MAM treated females performed better in a complex maze under lighted conditions than same-sex controls. Open field and running wheel activity levels in females were unaffected. Though not statistically significant, males exhibited a mild hyperactivity syndrome characterized by an increase in running wheel and open field activity, as well as a depressed startle response. Both sexes were hypersensitive to the locomotor-increasing effects of methamphetamine. These results suggest that the functional effects resulting from cerebellar hypoplasia produced by MAM treatment on PNDs 5-8 are milder than those resulting from MAM treatment beginning on the day of birth. The results are compared with other forms of cerebellar lesions and provide support for the hypothesis that early insult (day of birth or shortly after) produces hypoactivity whereas a later insult (day 4 or later after birth) produces a syndrome of hyperactivity.

Analysis of gene action in the meander tail mutant mouse: examination of cerebellar phenotype and mitotic activity of granule cell neuroblasts

The meander tail (mea) gene results in a stereotypic pattern of cerebellar abnormalities, most notably the virtual depletion of granule cells in the anterior lobe of the cerebellum. The causal basis of this mutation is unknown. In this paper we have taken a three-part approach to the analysis of mea gene action. First, we quantitatively determined the effect of the mea gene on granule cell and Purkinje cell number. We found, in addition to the marked depletion of anterior lobe granule cells ( > 90%), there were also significantly fewer granule cells in the posterior lobe (20-30%) without a concomitant loss of Purkinje cells. Second, we explored the relationship between granule cell depletion caused by the mea gene and by the mitotic poison, 5-fluoro-2'-deoxyuridine (FdU). Prenatal and postnatal ICR mice were treated with FdU to ascertain the regimen that best produces a meander tail-like cerebellar phenotype. The similarity of the effects of the mea gene and injections of FdU at E17 and PO suggests the hypothesis that the mea gene acts to disrupt the cell cycle of cerebellar granule cell precursors. Thus, the third part of this study was to test this hypothesis by using injections of either BrdU (5-bromo-2'-deoxyuridine) or 3H-thymidine into homozygous and heterozygous meander tail littermates at E17 or PO. After processing the tissue for BrdU immunocytochemistry or 3H-thymidine autoradiography, counts were made of the number of labeled and unlabeled external granule layer (EGL) cells to determine the percentage that had incorporated the mitotic label (labeling index). No difference in the labeling index was found between homozygous meander tail mice and normal, heterozygous littermate controls. Therefore, the mitotic activity of the EGL neuroblasts is not disrupted by the mea gene. Furthermore, while a mitotic poison can produce a phenotype similar to the action of the mea gene, mea is phenomenologically different from FdU treatment.

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.

Further evidence for a unique developmental compartment in the cerebellum of the meander tail mutant mouse as revealed by the quantitative analysis of Purkinje cells

The cerebellum of the meander tail mutant mouse (mea/mea) is characterized by a relatively normal cytoarchitecture posteriorly with an abrupt transition to an anterior region in which there is abnormal foliation, agranularity, and Purkinje cell (PC) ectopia. This study presents the results of a qualitative and quantitative analysis of the PC in the mea/mea cerebellum. Developmental and morphological analyses reveal that the PC in the anterior region of the mea/mea cerebellum do not form a monolayer during the first week of postnatal development as they do in the wild type mouse. In the adult mea/mea, the dendrites of these ectopic cells are atrophic and disoriented. Quantitative studies in adult animals reveal that while the total number of PC is normal, the number of PC in the affected anterior region of the mea/mea cerebellum is greater than the number of PC in the anterior lobe, as classically defined by the primary fissure, of the normal animal. These data suggest that 1) the developmental morphology of the PC in the anterior region is abnormal, probably due to the lack of granule cells at early postnatal times; 2) the total number of PC in the cerebellum is normal, and 3) the defect is not restricted to the anterior lobe but involves a portion of the posterior lobe. The latter supports the notion that the mutant gene affects a unique developmental compartment in the cerebellum which does not coincide with the classic adult boundary, the primary fissure, between the anterior and posterior lobes.

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.

Different climbing fibres innervate separate dendritic regions of the same Purkinje cell in hypogranular cerebellum

Electrophysiological experiments have shown that in hypogranular cerebella the Purkinje cells are innervated by several climbing fibres. The aim of this paper is to provide morphological evidence for this multiple innervation and to describe the topographical distribution of the different climbing fibres onto the somadendritic region of the Purkinje cell. Experiments have been performed in hypogranular adult Wistar rats lesioned during the first postnatal week by methylazoxymethanol (MAM) or by X-irradiation. Purkinje cells were labelled by an anti-calbindin antibody, whereas climbing fibres were visualised by means of Phaseolus vulgaris leucoagglutinin. Purkinje cells showed variable degrees of abnormality and displacement. Climbing fibres made contact with the dendrites of all kinds of Purkinje cells, including those ectopically positioned whose dendrites branched in the white matter. This shows that Purkinje cells can develop dendritic branching in the absence of granule cells and maintain the capability of interacting with their proper afferents, even when they are severely affected and displaced. In four Purkinje cells we have been able to follow the course of two climbing fibre terminal arbourisations. Almost no terminal branches were present around the Purkinje cell soma, and the whole arbour covered the proximal two-thirds of the Purkinje cell dendritic tree. These arbourisations, after an initial common course along the primary dendrite, distributed to separate dendritic regions. The observation of a single labelled climbing fibre covering a limited region of the dendritic tree was more common. As this finding is never observed in control material, it is concluded that the remaining region is covered by another unlabelled climbing fibre belonging to a different inferior olive neurone. These results represent a morphological demonstration of multiple climbing fibre innervation of the adult Purkinje cell. The maintenance of polyinnervation in the adult, which is consequent to the loss of granule cells, is not associated with a defect in the peridendritic translocation of the olivary arbour. In addition, the strict segregation of the different climbing fibres to distinct territories of the Purkinje cell dendritic tree suggests that each terminal arbourisation acts as a functionally independent unit and prevents other competitors from invading its own target domain.

Postnatal methylazoxymethanol: sensitive periods and regional selectivity of effects

Work on neonatal MAM exposure has focused primarily on exposure within the first week postpartum, and on resulting hypoplasia or stunting of the cerebellum. Rats in this study were exposed to MAM on 4 consecutive postnatal days (PND), beginning at one of six ages, from birth through weaning (PND 1, 5, 9, 13, 17, or 21). MAM was administered subcutaneously in doses of 3, 4, or 5 mg/kg twice per day. Rats were sacrificed at PNDs 28 or 84. The most sensitive age for MAM-induced stunting was determined to be PNDs 1-4. When 5 mg/kg MAM was administered twice daily on PNDs 1-4, body weight was reduced by 24% at age 28 days. Additionally, when compared to control rats, brains of the 28-day-old rats were stunted as follows: whole brain (11%), cerebellum (35%), hippocampus (11%), and olfactory bulb (27%). The effects of PND 1-4 MAM exposure were still evident at 84 days of age when cerebellum and olfactory bulbs from treated rats weighed 30% less than those same regions in control rats. These findings indicate that neonatal exposure to MAM results in permanent stunting in select regions of developing rat brain. This stunting, along with other known MAM effects, can be tailored by exposure age and dose to augment the use of MAM as a positive control for investigation of compounds with neurotoxic potential.

Prenatal neuroleptic exposure and growth stunting in the rat: an in vivo and in vitro examination of sensitive periods and possible mechanisms

There is increasing evidence that a number of neurotransmitters can play a trophic role in the development of the central nervous system. Dopamine is one candidate for this role. In a series of papers, Lewis, Patel, and colleagues have demonstrated that exposure to compounds which interfere with dopaminergic neurotransmission ("neuroleptics") can block cell proliferation in the brains of 11-day-old rat pups for at least 24 hr. More recently our laboratory has reported that prenatal exposure to haloperidol (HAL), a neuroleptic which binds to and blocks dopamine receptor sites in the adult brain, permanently stunts body and brain growth when that exposure extends throughout postimplantation pregnancy. Reported here are the results of two experiments conducted to further examine this phenomenon. The first experiment attempted to identify sensitive gestational periods for the HAL effect on growth in vivo. This experiment also assessed the effect of exposure to reserpine (RES), a compound which in the adult blocks dopaminergic neurotransmission by rupturing monoamine storage vesicles, an effect which is quite distinct from the HAL mechanism of action. In a second experiment, gestational day (GD) 9 embryos were exposed in vitro for 48 hr to either HAL, RES, or one of two specific blockers of dopamine receptor subtypes. Schering 23390 (SCH) was used as the D1 blocker, and sulpiride (SULP) as the D2 blocker. The in vivo experiment showed that twice-daily exposure to subcutaneous injections of HAL (5 mg/kg for each of the 2 injections) or RES (0.1 mg/kg for each injection) permanently stunted brain growth when injections were given in midpregnancy (GD 12-16), but not in late pregnancy (GD 16-20). RES was substantially more fetotoxic than HAL, especially late in pregnancy. The growth stunting produced by either compound with GD 12-16 exposure was not restricted to dopamine-rich areas of the brain, or indeed to the brain itself, in that body weight was also depressed. Pair-fed controls did not show the same magnitude or duration of stunting, indicating that this effect was not due to drug-induced maternal hypophagia. The in vitro experiment revealed that exposure to micromolar concentrations of any of the 4 neuroleptics reduced embryonic GD 11 DNA and protein content and delayed development. HAL and SCH had the most pronounced effects at concentrations close to blood levels reportedly produced by exposure to doses used in the in vivo experiments. RES was less potent, and SULP still less potent than RES.(ABSTRACT TRUNCATED AT 400 WORDS)

Eyeblink conditioning in the infant rat: an animal model of learning in developmental neurotoxicology

Classical conditioning of the eyeblink reflex is a relatively simple procedure for studying associative learning that was first developed for use with human subjects more than half a century ago. The use of this procedure in laboratory animals by psychologists and neuroscientists over the past 30 years has produced a powerful animal model for studying the behavioral and biological mechanisms of learning. As a result, eyeblink conditioning is beginning to be pursued as a very promising model for predicting and understanding human learning and memory disorders. Among the many advantages of this procedure are (a) the fact that it can be carried out in the same manner in both humans and laboratory animals; (b) the many ways in which it permits one to characterize changes in learning at the behavioral level; (c) the readiness with which hypotheses regarding the neurological basis of behavioral disorders can be formulated and tested; (d) the fact that it can be used in the same way across the life-span; and (e) its ability to distinguish, from normative groups, populations suffering from neurological conditions associated with impaired learning and memory, including those produced by exposure to neurotoxicants. In this article, we argue that these properties of eyeblink conditioning make it an excellent model system for studying early impairments of learning and memory in developmental neurotoxicology. We also review progress that has been made in our laboratory in developing a rodent model of infant eyeblink conditioning for this purpose.

Immunohistochemical localization of calbindin-D28K in telencephalic regions of microencephalic rats

The localization of calbindin neurons was studied in different brain areas of rats rendered microencephalic by gestational methylazoxymethanol acetate (MAM) treatment. In layers VI and V of the cortex, the only recognizable layers in MAM-treated rats, a higher density of calbindin interneurons and an apparent increase in protein expression was observed. In the hippocampus, calbindin pattern was essentially preserved, despite the dramatic decrease in size. In other telencephalic regions, calbindin distribution was not changed except for the septum, where a large increase of calbindin neurons was observed. The present results suggest that the MAM model may be used to investigate the role of calbindin.

Impaired neurogenesis by methylazoxymethanol in newborn rats results in transient reduction of ornithine decarboxylase and polyamines in the cerebellum, but not in the olfactory bulbs

Polyamines and the key enzyme for their biosynthesis, ornithine decarboxylase (ODC) play an important role in the control of neuronal proliferation and differentiation. Exposure to agents that interfere with normal cell maturation is expected to result in alteration of neuronal ODC developmental pattern. We have administered to newborn rats, about 6 and 30 hr after birth, 20 mg/kg of methylazoxymethanol acetate (MAM), an agent able to selectively kill dividing cells and we have evaluated ODC activity and polyamine levels in the cerebellum and ODC activity in the olfactory bulbs at various developmental stages starting from postnatal day 4 (PD 4) until PD 28. Cerebellar weight decreased by 22-50% at the different developmental stages in MAM-treated animals. A decline in ODC specific activity was observed at PD 4 and a decrease of putrescine levels at PD 4 and PD 6 in the cerebellum. At PD 10, however, both ODC activity and putrescine level were increased in MAM-treated animals. Spermidine levels were never affected by the treatment, while spermine was significantly decreased at PD 6 and PD 8. These results demonstrate that altered ontogenetic patterns of ODC activity and polyamine levels are the consequence of disturbance of the normal process of brain maturation. No significant differences in specific ODC activity were noticed in the olfactory bulbs of MAM-treated rats. This may be related to the more widespread time-span of neurogenesis in this region, a fact that is also revealed by the higher ODC activity constitutively expressed at times in which neurogenesis has ended in the rest of the brain.

Autoradiographic localization of [3H]-L-glutamate binding sites in a model of cerebellar granule cell ectopia generated by methylazoxymethanol treatment

The distribution of [3H]glutamate binding sites was studied in a model of altered cerebellar development obtained by injecting methylazoxymethanol (MAM) in 5-day-old mice. In these mice, at the 25th postnatal day, cerebella were smaller than normal, stratification was normal except for the presence in some lobes of a thin ectopic granule cell layer in the middle of the molecular layer, the proportion of the distribution of [3H]glutamate binding sites between molecular and internal granule cell layers was maintained but site density of both quisqualate- and NMDA-sensitive types was increased in the two layers. In the molecular layer, this increase was uniform in spite of the presence of the ectopic cell layer. In the internal granular layer, the increase of quisqualate-sensitive and NMDA-sensitive [3H]glutamate binding sites is topographically segregated and the first corresponds to areas of lesser cellular density. These results show that MAM treatment induces persistent alterations of the cerebellar glutamatergic system, which consist of receptor over-expression, possibly due to deficit of innervation, reactive gliosis and immaturity of surviving granule cells.

The fates of cells in the developing cerebral cortex of normal and methylazoxymethanol acetate-lesioned mice

We are interested in the mechanisms that generate the mature cerebral cortex. We used bromodeoxyuridine (BrdU) to label cortical cells as they were being born. We followed the fates of specific sets of cortical precursors in normal mice and in mice in which other groups of cortical progenitors had been destroyed with the antimitotic agent methylazoxymethanol acetate (MAM Ac). In normal mice, most cells destined for the cerebral cortex were produced from embryonic day 12 (E12) to E16 in the expected inside-to-outside sequence (deep layers first, superficial layers last). Injection of MAM Ac at E13 killed cells that would normally have contributed to the deep cortical layers. As a consequence, the cortex was thinned by approximately 25% at postnatal day 21 (P21). However, all laminae were present and had normal connections with subcortical structures, although all were proportionately thinner. BrdU injected on E16 labelled a normally sized complement of cells that spanned a larger proportion of the depth of the thinned cortex. Thus, the deep cortical layers comprised many cells that were born several days later than normal. At embryonic ages prior to E12, a transient set of cells is produced in the early telencephalon. After injection with MAM Ac at E10, the cortex appeared histologically and histochemically normal at P21. However, many cells that would normally have contributed to superficial cortex (born on E15) were significantly deeper than normal. These results suggest that, during the early stages of cortical development, the nervous system is sufficiently plastic to compensate to some extent for the destruction of specific precursor cells by altering the fates of neurons born later. They indicate that the embryonic date on which a cortical cell is born does not necessarily determine its eventual phenotype.

Prenatal haloperidol exposure: effects on brain weights and caudate neurotransmitter levels in rats

Monoamines may exert a trophic effect on early brain development. To assess the role of dopamine in prenatal neurological development of the rat, haloperidol (HAL) was given in daily 2.5 or 5 mg/kg SC doses to dams over gestational days 6 to 20. This treatment regime did not enhance fetal mortality, but did produce reliable, if modest, stunting of the body and brain weight of offspring. The 5 mg/kg HAL dose consistently reduced offspring brain weight to roughly 90% of controls. This effect was probably permanent, in that it was seen throughout maturation and in adults as late as 140 days of postnatal age. Appropriate controls showed that this effect was not due to drug-induced reductions in food intake, to the presence of HAL in maternal milk, or to behavioral abnormalities in HAL-exposed dams. These effects had, at best, modest regional specificity, in that most brain regions were affected, independently of degree of dopaminergic innervation. Closer investigation of HAL effects on the striatum suggested that this permanent weight reduction was not accompanied by alterations in striatal concentrations of monoamines, monoamine metabolites, amino acids, choline, acetylcholine, DNA, protein, or water. It is concluded that prenatal HAL does stunt growth, but that this effect may not involve a direct drug influence restricted to the fetal dopamine system in the brain.

Alteration of benzodiazepine receptors in mouse cerebellum following methylazoxymethanol treatment during development

The specific binding of [3H]flunitrazepam was studied to biochemically specify the morphological alterations induced in mouse cerebellum by a single injection of an antimitotic agent, methylazoxymethanol (MAM) performed at the beginning of the postnatal life. The MAM injection causes a general reduction of the benzodiazepine receptors in the adult mice which is particularly severe in mice having been injected the 1st day of postnatal life (so-called MAM0 mice) as compared to animals injected the 5th day (MAM5 mice): in MAM0 mice the benzodiazepine receptor is reduced to half of the control value. The affinity of the benzodiazepine towards its receptor was not affected and the topographic and biochemical action of MAM in the central nervous system was ascertained. Correlations could be made between the biochemical modifications and the morphological alterations otherwise described.

Regulation of granule cell number by a predetermined number of Purkinje cells in development

Development dysgenesis of Purkinje cells or granule cells was analyzed for the reciprocal effect of reduced number of each cell type on the other. A single pre- or postnatal injection of methylazoxymethanol acetate (MAM) in the rat reduces either the number of Purkinje cells or the number of granule cells when administered at the time of their respective genesis. The total number of these two types of neurons was obtained from cell density values of each layer and the total volume of the granular layer and the area of the Purkinje cell layer. The results show that Purkinje cells (targets) strictly determine the maximum number of granule cells (afferent neurons) following deficits in the number of Purkinje cells produced by prenatal MAM administration. Deficits in Purkinje cells were accompanied by a proportionally smaller number of granule cells so that the ratio remained constant. On the other hand, the reduction in the number of granule cells of the postnatal MAM model did not affect the number of Purkinje cells. These results indicate that the maximum number of these afferent neurons is constrained unidirectionally through a property defined by the number of their target neurons which develop earlier. Furthermore the number of afferent cells had no effect on the number of target cells.

Depletion of cholinergic habenulo-interpeduncular neurons by selectively timed methylazoxymethanol acetate (MAM) treatment during pregnancy

Methylazoxymethanol acetate (MAM) was injected to female rats at the beginning of the 17th day of gestation. Resulting offspring showed a remarkable decrease in the size of the medial habenula while the interpeduncular nucleus, whose neurons are generated before the time of MAM treatment, appeared anatomically unaffected. Choline acetyltransferase was significantly reduced in the habenulae and in the interpeduncular nucleus suggesting that MAM treatment had depleted a portion of the cholinergic neurons of the medial habenula which project to the interpeduncular nucleus. Aromatic amino acid decarboxylase significantly increased in the interpeduncular nucleus, a likely effect of monoaminergic hyperinnervation in response to partial cholinergic deprivation. MAM strategy can be usefully adopted for the study of general aspects of brain development when connected nuclei showing no overlapping in neuronal generation times are involved.

Developmental factors related to abnormal cerebellar foliation induced by methylazoxymethanol acetate (MAM)

Quantitation of mid-sagittal sections of the molecular layer, and both the external and internal granular layers between control and methylazoxymethanol acetate (MAM)-treated rats, at various stages of cerebellar development revealed a much smaller area of these layers in sagittal profile, however, the fiber core was not significantly affected by the drug. The expansion of the pial surface length was parallel to the length of the Purkinje cell layer, although comparison of a fissure index revealed hypofissuration in the experimental group. In histological examination, there was perforation, patching, and agenesis of the external granular layer. Mushroom expansion of the external granular layer occurred at patches producing a gyrating folial pattern rather than parallel ones. The number of lobules and their basic pattern remained normal. We conclude that the deficits in the external granular layer interrupted the growth force that produces the normal rostrocaudal organization of parallel coronal foliation and this resulted in shallow periodic fissuration along the sagittal extent. Fissurations forming lobules arose largely independent of the external granular layer by directed expansion of the central fiber core while normal parallel foliation is an elaboration of the lobular surface controlled by growth forces defined by both distribution of the external granular layer and the underlying fiber core with associated Purkinje cells.

Ectopic glial cells in rat cerebella following neonatal administration of methylazoxymethanol acetate

Immunological and ultrastructural studies of adult cerebella following neonatal injection of methylazoxymethanol acetate revealed ectopic glial cells in the molecular layer and at the pial surface. This finding strengthens the view that the external granular layer might give rise to Bergmann glia.

Perinatal methylazoxymethanol acetate uncouples coincidence of orientation of cerebellar folia and parallel fibers

Perinatal administration of methylazoxymethanol acetate in the rat, as a one time injection on gestational day 21, postnatal days 0, 1 or 2, altered the parallel orientation of cerebellar folia. The effect persisted into adulthood. In animals injected on one of the postnatal days 3, 4 or 5, the folial pattern was not altered. Even when the injection was repeated for three days on postnatal days 3, 4 and 5, changes in the cerebellar surface were not found. However, in animals receiving a low protein diet during the last five days of gestation, the three injection regimen produced a distortion of the folial pattern. The surface of cerebella of animals injected on gestational day 21 through postnatal day 2 was covered with small blebs resembling the surface of a cauliflower head. In sagittal sections, islands of cortical laminae appeared to be isolated from the arbor vitae. However, serial reconstruction of the granular layer from sections revealed that these pieces were continuous with the arbor vitae. Surprisingly, cerebella having malaligned folia also had varying degrees of Purkinje cell somas distributed throughout the granule cell layer rather than in a single layer. This occurred even when the granule cell layer approached normal thickness. Analysis of cerebellar weight from the group injected on the day of birth revealed three levels of weight reduction: severe (greater than 40%), moderate (20-40%) and mild (less than 20%). The granule cell deficit was directly related to the weight reduction of the cerebella. In the severely-affected cerebella, areas of the cortex were virtually devoid of granule cells. The moderately-affected cerebella had a continuous granular layer which was thick and thin. In the mild type, the layer was relatively normal in thickness but, nevertheless, the cerebellar surface was highly distorted. In all animals treated with methylazoxymethanol acetate on days G21 through P5, parallel fibers were disoriented. This occurred even though the folia appeared normal in the G20, P3, P4, P5 and P3-5 injected groups. Bundles of parallel fibers crisscrossed in the plane of the cerebellar surface in all areas where a molecular layer was found.(ABSTRACT TRUNCATED AT 400 WORDS)

The angiogenesis of the micrencephalic rat brains caused by methylazoxymethanol acetate. III. Internal angioarchitecture of cortex

The intracortical angioarchitecture of normal and micrencephalic rat brains was examined. The neuroblast migration was disturbed by injection of the neurotoxin methylazoxymethanol acetate, administered on day E14. The internal vascularization of the malformed cortex showed severe damage to the layered distribution of vascular trunks in contrast to controls. A pathological course and marked variability in the density of the radial vessels were seen in the parieto-occipital areas, in which the neuroblast migration was most severely affected. These observations show the decisive role of neuroblast migration and maturation in the development of the cortical angioarchitecture.