Direct and indirect effects on the lateral geniculate nucleus neurons of prenatal exposure to methylazoxymethanol acetate

A neurobehavioral systems analysis of adult rats exposed to methylazoxymethanol acetate on E17: implications for the neuropathology of schizophrenia

BACKGROUND

As a test of plausibility for the hypothesis that schizophrenia can result from abnormal brain, especially cerebral cortical, development, these studies examined whether, in the rat, disruption of brain development initiated on embryonic day (E) 17, using the methylating agent methylazoxymethanol acetate (MAM), leads to a schizophrenia-relevant pattern of neural and behavioral pathology. Specifically, we tested whether this manipulation leads to disruptions of frontal and limbic corticostriatal circuit function, while producing schizophrenia-like, region-dependent reductions in gray matter in cortex and thalamus.

METHODS

In offspring of rats administered MAM (22 mg/kg) on E17 or earlier (E15), regional size, neuron number and neuron density were determined in multiple brain regions. Spontaneous synaptic activity at prefrontal cortical (PFC) and ventral striatal (vSTR) neurons was recorded in vivio. Finally, cognitive and sensorimotor processes mediated by frontal and limbic corticostriatal circuits were assessed.

RESULTS

Adult MAM-E17-exposed offspring showed selective histopathology: size reductions in mediodorsal thalamus, hippocampus, and parahippocampal, prefrontal, and occipital cortices, but not in sensory midbrain, cerebellum, or sensorimotor cortex. The prefrontal, perirhinal, and occipital cortices showed increased neuron density with no neuron loss. The histopathology was accompanied by a disruption of synaptically-driven "bistable membrane states" in PFC and vSTR neurons, and, at the behavioral level, cognitive inflexibility, orofacial dyskinesias, sensorimotor gating deficits and a post-pubertal-emerging hyper-responsiveness to amphetamine. Earlier embryonic MAM exposure led to microcephaly and a motor phenotype.

CONCLUSIONS

The "MAM-E17" rodent models key aspects of neuropathology in circuits that are highly relevant to schizophrenia.

The distribution of serotonergic nerves in microencephalic rats treated prenatally with methylazoxymethanol

Prenatal exposure of pregnant rats to methylazoxymethanol acetate (MAM) induces microencephaly in the offspring. In the present study of these microencephalic rats (MAM rats) we used quantitative autoradiography to investigate [3H] paroxetine binding sites, which are a selective marker of serotonin (5-HT) transporters (5-HTT). The binding in the accumbens, cortex, hippocampus, and dorsolateral thalamus was significantly increased in MAM rats, compared to the control rats, while there was a significant decrease in the dorsal raphe nucleus of the MAM rats. The levels of 5-HTT mRNA in the dorsal raphe nuclei were analyzed by in situ hybridization, which revealed a significant decrease in 5-HTT mRNA-positive neurons in the MAM rats compared to the control rats. The results imply serotonergic hyperinnervation in the cerebral hemispheres of MAM rats, while a target-dependent secondary degeneration of 5-HT neurons might be induced in the dorsal raphe nuclei of MAM rats.

Neurogenesis and commitment of corticospinal neurons in reeler

In the homozygous (but not the heterozygous) reeler mutant, disruption of neuron migration leads to a major perturbation of the cortical environment that in turn could modify (1) the specification of neuronal fate and (2) the proliferation dynamics of cortical precursors. To investigate these issues, tritiated thymidine injections during cortical neurogenesis were coupled with postnatal injections of a retrograde tracer in the spinal cord to accurately measure the neurogenesis of corticospinal neurons in the heterozygous and homozygous mutant. The homozygous reeler shows (1) strict conservation of area-specific timetables of corticospinal neuron generation; (2) neurons with the appropriate birthdates show an enhanced probability of projecting to the spinal cord; (3) during early stages of corticogenesis, there is a reduced rate of neuron production followed at later stages by an increased rate of neuron production; and (4) these changes in the rate of neuron production were shown to be at least partially attributable to changes in the proportions of differentiative divisions. Taken together, our results show that in the developing cortex, the neurogenesis and specification of a given neuronal phenotype are partially controlled by the postmigratory compartment. On the other hand, neither areal identity nor the chronology of production of layer-specific neuronal phenotype seems to depend on the integrity of the cellular environment.

Methylazoxymethanol acetate-induced abnormalities in the entorhinal cortex of the rat; parallels with morphological findings in schizophrenia

It has been suggested repeatedly that the non-heritable factors in the pathogenesis of schizophrenia involve abnormalities of prenatal neurodevelopment. Furthermore, post-mortem studies show neuropathology of apparently developmental origin in the entorhinal cortex and other brain regions of schizophrenic subjects. In an attempt to model a developmental defect of the entorhinal region in the rat, cerebrocortical proliferation was briefly interrupted during its earliest stages, when the entorhinal area is thought to undergo major cell division. Specifically, the experimental set-up involved the administration of methylazoxymethanol acetate (MAM) on 1 of 4 consecutive days of embryonal development, from E9 to E12. Analysis of the forebrain in adult animals shows reduction of the entorhinal cortex in rats treated on each of these days. This effect shifts from lateral to medial divisions of the entorhinal cortex with later administration of MAM, following a known developmental gradient. Morphological consequences of MAM administration appear to be largely confined to the entorhinal cortex in the groups treated on E9 to E11, although slight reductions of the frontal and occipital neocortex were also observed in these animals. MAM treatment on E12 produces relatively more widespread damage, as reflected among other in a small reduction of brain weight. The described brain abnormalities are not accompanied by obvious phenotypical changes in any, but the E12-treated group. They, moreover, involve cortical thinning, disorganised cortical layering, and abnormal temporal asymmetries. These finding bare some similarity to observations in brains of schizophrenic subjects. The possible relevance of this approach in modeling neurodevelopmental aspects of schizophrenia is discussed.

Radiation-induced, lamina-specific deletion of neurons in the primate visual cortex

To examine the early determinants of cortical cytoarchitecture, we deleted specific neuronal classes in the primate visual cortex by ionizing irradiation at selected prenatal stages. Multiple doses of X-rays were delivered to the macaque monkey brain between embryonic day (E) 80 and E90 to block the division of cells destined to populate the superficial cortical layers, between E70 and E79 to eliminate neurons destined for the middle layers; and between E33 and E40 to delete neurons destined for the lateral geniculate nucleus (LGN) that project to the cortex. All animals were killed after birth, and their brains were processed for histological and electron microscopic analyses. Cell density and number in the LGN and visual cortex were determined by using three-dimensional, computer-aided morphometry. In animals irradiated with low doses (total of approximately 200 cGy) during the genesis of the LGN but before the onset of corticogenesis (E33-40), the LGN was reduced in both volume and number of neurons. Area 17 in these animals displayed only slight changes in cortical thickness, cell density, and area-specific cytoarchitectonic features, whereas the total surface devoted to area 17 was significantly diminished. In contrast, animals irradiated with low doses during the period of corticogenesis, after the completion of the LGN genesis, showed no significant change in the volume of the LGN or in the number of its cells. Moreover, in these animals, the surface of area 17 was not significantly altered, although the cortical layers generated at the time of irradiation had a significantly lower density and total number of cells, whereas the layers generated before and after the period of irradiation were spared. In contrast, cases exposed to high doses of X-ray (total > 300 cGy) showed more severe effects, including all layers. However, layers normally generated during irradiation were depleted and consisted of cell-sparse strata populated by densely packed neuropil (axons, small dendrites, dendritic spines, and synaptic boutons). These cell-sparse strata were situated deeper in the early irradiated animals than in the later irradiated animals, and their laminar position changed abruptly at the area 17/18 border. These results show that low doses of irradiation in a slowly developing primate brain can be used effectively to eliminate targeted classes of neurons before they reach their final position, providing an opportunity to examine the role of cell-cell interactions in the formation of circuitry and the role of specific cell classes in cortical development.

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.

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.

Developmental alteration of serotonin neurons in the raphe nucleus of rats with methylazoxymethanol-induced microcephaly

Prenatal exposure of pregnant rats to methylazoxymethanol acetate (MAM), an anti-mitotic agent, on day 15 of gestation induces severe microcephaly in the offspring. The present study first investigated a developmental alteration of serotonin (5HT) neurons immunohistochemically in the dorsal and median raphe nuclei in serial sections in both control and microcephalic rats (MAM-rats) at 35 days of age. 5HT-immunoreactive neurons in the MAM-rats were reduced in number and irregularly distributed in the dorsal and median raphe nuclei compared with those in the control. The dendrites of neurons in these nuclei in the MAM-rats were very short and twisted. A follow-up observation on the development of the cerebral cortex at 5, 9 and 28 days of age was performed using Nissl-stained preparations, which revealed a disorganized cell arrangement in the cerebral cortex of the MAM-rats at the very early postnatal period. Furthermore, the distribution of 5HT-immunoreactive fibers into the cerebral cortex was also examined using brains of 28 days of age. In MAM-rats of this age, abnormally tortuous 5HT-immunoreactive fibers were observed in the cerebral cortex. 5HT neurons in the raphe nuclei are known to project their ascending axons widely into the entire cortical area during the 1st postnatal week. Thus, the association of disorganized cortical cell arrangement and the hyperdense and tortuous distribution of 5HT-immunoreactive fibers in the cerebral cortex support the idea of target-dependent secondary degeneration of 5HT neurons in the dorsal and median raphe nuclei of the MAM-rats.

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.

Visual discrimination by rats with transplacentally induced micrencephaly

Long-Evans rats with micrencephaly induced by prenatal exposure to methylazoxymethanol acetate and normal controls were trained in a two-choice box to discriminate between stimuli of different brightness (black vs. white) or pattern (horizontal vs. vertical alternating black-and-white stripes). Mild footshock was used to motivate the rats to learn. The micrencephalic rats were impaired in learning the pattern, but not brightness discrimination. These results confirm and extend similar findings with micrencephalic Wistar rats by another laboratory. The visual discrimination performance of micrencephalic rats was similar to that reported for normal rats with lesions in the visual cortical areas.

Differential target dependence in the developing brain: implications for mental retardation

During development of the brain, many neurons exhibit a dependence on other neuronal populations for their survival and differentiation (target dependence). Evidence suggests that some neural pathways are much more dependent on single target neuronal populations for their survival than are others (differential target dependence). This phenomenon has important implications both for animal models of congenital human brain damage and for ideas concerning the aetiology of behavioural abnormalities associated with human mental retardation. Predictions of the neuronal deficits likely to arise from exposure to cytotoxic agents (e.g. ionizing radiation, hyperthermia, viral infection) at a particular time must take differential target dependence into account. It is known that target dependence affects corticopetal pathways involved with the discriminative senses (e.g. vision), more than monoaminergic and cholinergic corticopetal pathways which are believed to be involved with arousal, selective sensory attention, sleep, memory and cortical vasomotor function. Following prenatal damage to superficial layers of the cerebral cortex, this effect of differential target dependence leads not only to a relative hyperinnervation of the cortex with monoaminergic and cholinergic projections, but a specific deficit in visual pathways. The implications of this combined deficit for the behaviour and rehabilitation of the mentally retarded are considered.

Patterns of transiently expressed acetylcholinesterase activity in cerebral cortex and dorsal thalamus of developing rats with cytotoxin-induced microencephaly

Previous studies have demonstrated that acetylcholinesterase (AChE) activity is expressed transiently by thalamocortical neurons of primary sensory systems in developing rat pups. In the present study, prenatal treatment with methylazoxymethanol acetate (MAM) on embryonic day 15, 16, or 17 resulted in rat pups with cerebral cortices markedly reduced in thickness and areal extent. Histochemical studies demonstrated that AChE staining occurs in fiber-like plexuses in primary visual, auditory, and somatosensory regions of developing cerebral cortex of MAM-treated animals, just as in normal developing rats, but that the transient patterns of AChE are found more superficially than normal and they occur in an abnormal patchy distribution. Neuronal somata in thalamic lateral geniculate, medial geniculate and ventral basal nuclei of MAM-treated animals show transient AChE staining indistinguishable from that seen in normal animals. These data indicate: (1) AChE is expressed transiently by thalamocortical neurons in MAM-treated animals, (2) intensity of the transiently expressed AChE is not affected by MAM-induced loss of cortical neurons, and (3) the abnormal AChE patterns in cortex likely reflect the abnormal distributions of thalamocortical terminal fields that are characteristic of MAM-treated animals.

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.

Neocortical transplants in the micrencephalic rat brain: morphology and behavior

Normal fetal (E18) neocortical tissue transplanted into the hypoplastic posterior neocortex of infant (10 +/- 2-day-old) rats with transplacentally induced micrencephaly developed into very large, healthy, and permanent transplants. Although the cellular organization within the transplants rarely resembled that of normal rat neocortex, the transplants formed a broad area of interface with the host brain and established fiber connections with it. When tested at 2 months and 1-year-of-age, the presence of the transplant had no significant effect on the typically abnormal performance of micrencephalic rats on two tests of unspecific function, open field activity and maze learning. However, a small group of micrencephalic rats in whom the transplant tissue had failed to fill in the small brain lesions inescapably inflicted during surgery, showed greater behavioral deficits than the micrencephalic controls, suggesting that the transplant had corrected the lesion effect.