Hyperactivity and instrumental learning deficits in methylazoxymethanol-treated rat offspring

Characterization of heterotopic cell clusters in the hippocampus of the rat after prenatal treatment of methylazoxymethanol acetate

Prenatal exposure of methylazoxymethanol acetate, a DNA methylating agent, to pregnant rats on embryonic day 15 is known to produce hippocampal malformation and laminar disorganization of the cerebral cortex. However, there are few studies to demonstrate developmental processes of abnormal structures in the hippocampus. In the present study, we examined complete serial sections of rat brains on postnatal day 0 to 2, which pretreated with methylazoxymethanol acetate on embryonic day 15. At birth, massive cellular clusters were found under the white matter of the cerebral cortex and then, a part of these clusters entered into the hippocampal CA1 sector on postnatal day 2. These ectopic cellular clusters in the CA1 were immunoreactive to anti-calbindin antibody, suggesting that the origin of these cellular clusters is equivalent to that of the cortical layer II/III neurons. Next, we injected FluoroGold into the lateral septal nucleus to examine hippocampo-septal projection. FluoroGold-labeled neurons were scattered in the ectopic cellular cluster, implying that CA1 pyramidal neurons project normally to the lateral septal nucleus. In conclusion, a majority of neurons found in the ectopic cellular cluster caused by prenatal methylazoxymethanol treatment is derived from cortical neurons, and some intrinsic pyramidal neurons in the CA1 of hippocampus are scattered throughout the ectopic cellular cluster.

Developmental learning impairments in a rodent model of nodular heterotopia

Developmental malformations of neocortex—including microgyria, ectopias, and periventricular nodular heterotopia (PNH)—have been associated with language learning impairments in humans. Studies also show that developmental language impairments are frequently associated with deficits in processing rapid acoustic stimuli, and rodent models have linked cortical developmental disruption (microgyria, ectopia) with rapid auditory processing deficits. We sought to extend this neurodevelopmental model to evaluate the effects of embryonic (E) day 15 exposure to the anti-mitotic teratogen methylazoxymethanol acetate (MAM) on auditory processing and maze learning in rats. Extensive cortical anomalies were confirmed in MAM-treated rats post mortem. These included evidence of laminar disruption, PNH, and hippocampal dysplasia. Juvenile auditory testing (P21–42) revealed comparable silent gap detection performance for MAM-treated and control subjects, indicating normal hearing and basic auditory temporal processing in MAM subjects. Juvenile testing on a more complex two-tone oddball task, however, revealed a significant impairment in MAM-treated as compared to control subjects. Post hoc analysis also revealed a significant effect of PNH severity for MAM subjects, with more severe disruption associated with greater processing impairments. In adulthood (P60–100), only MAM subjects with the most severe PNH condition showed deficits in oddball two-tone processing as compared to controls. However, when presented with a more complex and novel FM sweep detection task, all MAM subjects showed significant processing deficits as compared to controls. Moreover, post hoc analysis revealed a significant effect of PNH severity on FM sweep processing. Water Maze testing results also showed a significant impairment for spatial but not non-spatial learning in MAM rats as compared to controls. Results lend further support to the notions that: (1) generalized cortical developmental disruption (stemming from injury, genetic or teratogenic insults) leads to auditory processing deficits, which in turn have been suggested to play a causal role in language impairment; (2) severity of cortical disruption is related to the severity of processing impairments; (3) juvenile auditory processing deficits appear to ameliorate with maturation, but can still be elicited in adulthood using increasingly complex acoustic stimuli; and (4) malformations induced with MAM are also associated with generalized spatial learning deficits. These cumulative findings contribute to our understanding of the behavioral consequences of cortical developmental pathology, which may in turn elucidate mechanisms contributing to developmental language learning impairment in humans.

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.

Effects of prenatal methylazoxymethanol acetate (MAM) treatment in rats on water maze performance

Prenatal methylazoxymethanol acetate (MAM) treatment has been shown to induce morphological abnormalities in cortical areas of the offspring. Based on the neuroanatomical and behavioural abnormalities, this treatment has been suggested as a useful animal model for schizophrenia. In a previous study (Jongen-Relo AL, Leng A, Luber M, Pothuizen HHJ, Weber L, Feldon J. The prenatal methylazoxymethanol acetate treatment: a neurodevelopmental animal model for schizophrenia? Behav Brain Res 2004;149:159-81) we have studied MAM-treated animals in a series of behavioural tests related to schizophrenia, such as latent inhibition and pre-pulse inhibition of the acoustic startle response to establish the validity of prenatal MAM treatment (20mg/kg i.p. on gestational days 9-15; MAM 9-MAM 15). We found that, apart from a marginal effect of increased activity in the open field, the MAM treatment on gestational day 15 was behaviourally ineffective. Here, we extended our previous study to a water maze experiment conducted in the same batch of animals as presented previously (MAM 12-MAM 15). MAM-treated animals showed similar water maze performance compared with control animals during the acquisition phase and the probe tests. However, during the reversal phase, MAM 15 animals showed impaired acquisition of the new platform location. This might indicate some cognitive deficits in MAM 15 animals in terms of working memory or behavioural flexibility. However, in combination with the lack of behavioural abnormalities of MAM 12-MAM 15 animals in several other tests related to schizophrenia in the previously reported study, the use of MAM treatment (MAM 12-MAM 15) as a valid model for schizophrenia still remains debatable.

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.

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.

Prenatal coexposure to metallic mercury vapour and methylmercury produce interactive behavioural changes in adult rats

Pregnant rats were 1) administered methyl mercury (MeHg) by gavage, 2 mg/kg/day during days 6-9 of gestation, 2) exposed by inhalation to metallic mercury (Hg degrees) vapour (1.8 mg/m3 air for 1.5 h per day) during gestation days 14-19, 3) exposed to both MeHg by gavage and Hg degrees vapour by inhalation (MeHg + Hg degrees), or 4) were given combined vehicle administration for each of the two treatments (control). The inhalation regimen corresponded to an approximate dose of 0.1 mg Hg degrees/kg/day. Clinical observations and developmental markers up to weaning showed no differences between any of the groups. Testing of behavioural function was performed between 4 and 5 months of age and included spontaneous motor activity, spatial learning in a circular bath, and instrumental maze learning for food reward. Offspring of dams exposed to Hg degrees showed hyperactivity in the motor activity test chambers over all three parameters: locomotion, rearing and total activity; this effect was potentiated in the animals of the MeHg + Hg degrees group. In the swim maze test, the MeHg + Hg degrees and Hg degrees groups evidenced longer latencies to reach a submerged platform, which they had learned to mount the day before, compared to either the control or MeHg groups. In the modified, enclosed radial arm maze, both the MeHg + Hg degrees and Hg degrees groups showed more ambulations and rearings in the activity test prior to the learning test. During the learning trial, the same groups (i.e., MeHg + Hg degrees and Hg degrees) showed longer latencies and made more errors in acquiring all eight pellets. Generally, the results indicate that prenatal exposure to Hg degrees causes alterations to both spontaneous and learned behaviours, suggesting some deficit in adaptive functions. Coexposure to MeHg, which by itself did not alter these functions at the dose given in this study, served to significantly aggravate the changes.

Potentiating effects of methamphetamine on the hyperactivity of microencephalic rats treated prenatally with methylazoxymethanol: possible implication of hyperdopaminergia

Microencephalic rats induced by methylazoxymethanol (MAM) were observed to have notable hyperactivity compared to control rats, as measured by several behavioral parameters in an automated field apparatus. Acute injection of the stimulant drug, methamphetamine (MAP), produced an increase in the incidence of locomotion and rearing in control rats, and this stimulatory effect of MAP on motor activity was markedly potentiated in MAM rats. Chronic MAP treatment did not change D1 or D2 dopamine receptor densities in either control or MAM rats. From these results, it was suggested that augmented dopaminergic functions may contribute to the hyperactivity seen in MAM rats.

Effects of methylazoxymethanol-induced micrencephaly on temporal response differentiation and progressive ratio responding in rats

Micrencephalic Sprague-Dawley rats were produced by an injection of 20 mg/kg methylazoxymethanol acetate on gestational Day 14. Brain weights of the offspring were 70% of controls while weights of frontal cortex and hippocampus were approximately 58% (Ferguson, Racey, Paule, & Holson, 1993). Operant performance was measured with particular emphasis on assessment of time estimation. The temporal response differentiation (TRD) and the progressive ratio (PR) tasks, previously used in the NCTR operant test battery for monkeys, were chosen for evaluation. The TRD schedule is notably different from other temporal tasks in that it requires subjects to initiate and maintain a lever press for 10-14 s. The PR task was included as a measure of motivation to work for food reinforces. Micrencephalics acquired and performed both tasks comparably to controls. During extinction, however, micrencephalics exhibited an increased TRD lever hold duration. This suggests an atypical response perservation, that is, perseverating the previously correct response. Previously, frontal cortical alterations were suggested to contribute heavily to micrencephalic-induced behavioral alterations (Ferguson et al., 1993). This study provides further evidence that response perseveration, a hallmark of frontal cortical lesions, is expressed in micrencephalic rats.

Behavioural effects of prenatal metallic mercury inhalation exposure in rats

The effects of administration by inhalation of metallic mercury vapour (Hg0) to pregnant rats, approximately corresponding to doses of 0.2 mg Hg0/kg/day (high dose) or 0.07 mg Hg0/kg/day (low dose), on the developmental and behavioural repertoire of the offspring were studied. Exposure occurred during days 11-14 plus 17-20 of gestation. The dose levels were selected so as not to induce maternal toxicity. Maturation variables such as surface righting, negative geotaxis, pinna unfolding, and tooth eruption revealed no differences between Hg0-treated offspring and controls. Tests of spontaneous motor activity showed that the Hg0-treated offspring were hypoactive at 3 months of age but hyperactive at 14 months. In spatial learning tasks the prenatally exposed offspring showed retarded acquisition in the radial arm maze but no differences in circular swim maze. A simple test of learning, habituation to a novel environment (activity chambers), indicated a reduced ability to adapt. These data suggest that prenatal exposure to Hg0 vapour results in similar behaviour changes in the offspring as reported for methylmercury.

Effects of foetal treatment with methylazoxymethanol on noradrenergic synapses in rat cerebral cortex

Methylazoxymethanol (MAM)-induced cerebral hypoplasia resulted in a significant increase in densities of norepinephrine uptake sites in cerebral cortex, suggesting that norepinephrinergic axon terminals were compressed in the smaller brain volumes. The density of beta-adrenergic receptors in MAM-lesioned cerebral cortex was decreased probably due to down-regulation, while there were no changes in the proportions and affinities of agonist high-affinity sites and agonist low-affinity sites in the desensitized beta-receptors.

Behavioural effects of neonatal metallic mercury exposure in rats

The effect of neonatal exposure of rats to mercury vapour (Hg0), at the concentration 0.05 mg/m3, 1 h (low dose) or 4 h (high dose), on the behaviour in adulthood were studied. Exposure occurred on days 11-17 (the period of rapid brain growth). Tests for spontaneous motor activity were performed at the ages of 2 and 4 months. Rats exposed to the high dose Hg0 showed a marked increase in variables locomotion and total activity but a decrease for rearing when tested at 2 months of age. At 4 months of age these rats showed a marked hypoactivity with respect to all three variables. Rats exposed to the low dose showed no significant differences at 2 months compared to controls. However, at the age of 4 months the same pattern (increase in variables locomotion and total activity but a decrease for rearing) already noticed in the high dose group at 2 months was observed. In the spatial learning tasks applied, the radial arm maze and circular swim maze, neonatally exposed pups showed a retarded acquisition to the former, while there was no difference compared to controls in the latter. These data indicate that neonatal exposure to mercury vapour results in similar behaviour changes as reported from offspring prenatally exposed to mercury vapour or methylmercury. Furthermore, exposure for 1 week to concentrations around Swedish threshold values (TLV) for 1 or 4 h resulted in dose and age-related behavioural changes.

Excitatory amino acid receptors in brains of rats with methylazoxymethanol-induced microencephaly

We used methylazoxymethanol-acetate (MAM), a potent alkylating agent, to produce microencephaly in offspring by injecting it into pregnant rats on day 15 of gestation. Binding activities of central excitatory amino acid receptors were examined in Triton-treated membranes prepared from brains of adult offspring with MAM-induced microencephaly (MAM rats). MAM rats exhibited approximately 40-50% reductions of the wet weights of the cerebral cortex, hippocampus and striatum compared to those in controls. In the cortex and hippocampus of MAM-rats, total bindings of [3H]glutamate (Glu) (which is sensitive to N-methyl-D-aspartate (NMDA) receptor), and strychnine-insensitive [3H]glycine (Gly) and (+)-5-[3H]methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801; a noncompetitive antagonist of NMDA receptor), were reduced to approximately 40% of those in controls. Similarly, in both regions of MAM rats, total bindings of [3H]kainate and DL-alpha-amino-3-[3H]hydroxy-5-methylisoxazole-4-propionic acid (an agonist of quisqualate receptors), were reduced to approximately 35-50% of those in controls. However, total bindings of these radioligands in the striatum of MAM rats were more than 65% of those in controls, despite the significant loss of striatum mass. However, specific bindings of radioligands in the striatum of MAM rats were elevated by more than 60% of those in controls, and Scatchard analysis revealed that elevations of [3H]Glu, [3H]Gly and [3H]MK-801 bindings were due to a significant increase in the densities of binding sites, with their affinities remaining unaltered. Spatial recognition ability examined by an 8-armed radial maze task was markedly impaired compared to those in controls. These results suggest that the proliferation of neurons bearing excitatory amino acid receptors (EAA) in the striatum is less affected by MAM treatment on day 15 of gestation than that in the cortex and hippocampus in spite of drastic weight loss in these brain regions. The significant reduction of EAA receptors in the cortex and hippocampus may be involved in the impairment of spatial memory observed in MAM-treated rats.

Effects of fetal treatment with methylazoxymethanol acetate on radial maze performance in rats

Pregnant rats (Wistar-Imamichi strain) were treated with 15 mg/kg/d of methylazoxymethanol acetate (MAM) on days 13-15 of gestation. Nine male rats, which were randomly selected from the MAM-treated offspring (MAM rats), were examined for their spatial recognition ability by the radial maze technique and compared with control offspring. Although the performances of MAM rats were inferior to the control, they could reach the predetermined criterion within 15 trials. Subsequent retention tests revealed the drastic impairment of performance in MAM rats when the retention interval was over 15 min. The total activity of choline acetyltransferase showed a significant decrease in the hippocampus and cerebral cortex of MAM rats. These results suggest that working memory disorders of MAM rats on radial maze tasks may be due to the lowering of cholinergic functions in their hippocampus and cerebral cortex.

Effects of prenatal exposure to tributyltin and trihexyltin on behaviour in rats

The effects of prenatal administration of tributyltin (1 and 5 mg/kg) and trihexyltin (5 mg/kg) upon the development and behavioural repertoire of rats were studied. The dose levels were selected so as not to induce maternal toxicity. No consistent delay upon occurrence of various maturation markers of the organotin-treated offspring was seen. As adults the tributyltin-treated offspring showed considerable hyperactivity following the initial habituation whereas the trihexyltin-treated offspring showed hyperactivity to a lesser degree. In the spatial learning tasks applied, the radial arm maze and the circular swim maze, tributyltin-treated rats demonstrated a clearly retarded aquisition of the radial arm maze task whereas trihexyltin-treated rats performed as well as the control rats; no differences were obtained in the swim maze task. The tributyltin-treated offspring showed a drastic potentiation of d-amphetamine-induced hyperactivity, whereas trihexyltin treatment induced only a marginal increase.

Increased uptake sites for serotonin and dopamine with decreased S2 serotonin receptors in microencephalic rat brain

Methylazoxymethanol (MAM)-induced cerebral hypoplasia resulted in a significant increase in densities of both serotonin uptake sites in frontal cortex and dopamine uptake sites in striatum, suggesting serotonergic and dopaminergic axons terminals were compressed in the smaller brain volumes. The density of S2 serotonin receptors in MAM-lesioned frontal cortex was decreased probably due to down-regulation, while densities of D1 and D2 dopamine receptors in striatum were identical between MAM-lesioned rats and control rats.

Prenatal methylazoxymethanol treatment potentiates d-amphetamine- and methylphenidate-induced motor activity in male and female rats

The effects of the stimulant drugs, d-amphetamine and methylphenidate, upon the motor activity of male and female off-spring of pregnant rats, treated on gestation day 15 with the antimitotic agent methylazoxymethanol (MAM, 25 mg/kg) were studied in four experiments. Cortical and striatal hypoplasia induced by prenatal administration of MAM resulted in increased concentrations of catecholamines in those regions. Administration of d-amphetamine and methylphenidate caused significant increases in motor activity; this effect was markedly potentiated in the MAM-treated rats, both the male and female off-spring. Thus, the locomotion and total activity parameters showed similar, but not identical, drastic increases in behaviour induced by the stimulant drugs as a result of the prenatal MAM treatment whereas for the rearing parameter a lesser potentiation by the MAM treatment was observed. This potentiation of the excitatory effects of the stimulant compounds upon the behavioural parameters is interpreted in terms of a relative increase in the density of catecholaminergic terminals in the forebrain regions of the central nervous system. The present results are discussed with regard to the utility of prenatal MAM treatment as a possible animal model for certain neurological disorders.