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

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.

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.

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.

Nocturnal hyperactivity induced by prenatal methylazoxymethanol administration as measured in a computerized residential maze

Treatment of female Sprague-Dawley rats with 15 or 25 mg/kg (IP) of methylazoxymethanol acetate (MAM) at gestational day 15 (15 DG) resulted in a dose-dependent reduction of total brain weight of the adult offspring. When tested for spontaneous activity in a residential maze over a 23 hour period, those animals treated with the highest dose of MAM showed an increase in both locomotion and local activity during night hours without changes in the structure of behavior. Animals treated with 15 mg/kg of MAM showed no difference in activity compared to controls despite a significant reduction in brain weight.

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.

Hyperactivity and instrumental learning deficits in methylazoxymethanol-treated rat offspring

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

Brain changes in rats induced by prenatal injection of methylazoxymethanol

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

Impaired selective attention in methylazoxymethanol-induced microencephalic rats

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

Effects of alpha-fluoromethylhistidine on sleep-waking parameters in rats

Effects of alpha-fluoromethylhistidine (FMH), an irreversible inhibitor of histidine decarboxylase, on the sleep-waking parameters were studied in rats for 24 hours. Intraperitoneal administration of FMH (100 mg/kg) at 11:30 hr resulted in a longer sleep latency compared with the control values. Hour-to-hour analyses revealed that wakefulness (W) time decreased (from 20:00 to 07:00 hr) and slow wave sleep (SS) time increased (from 19:00 to 06:00 hr) in the night. Paradoxical sleep (PS) time did not parallel the SS changes; it was increased significantly from 07:00 to 11:00 hr in the next morning. The influence of FMH seemed to be divided into direct, immediate action (increase of W) and late, prolonged action (decrease of W), and the results obtained support the histamine arousal hypothesis.

EEG and seizure threshold in normal and lissencephalic ferrets

Changes in EEG and susceptibility to electrically induced seizures were examined in the ferret with lissencephaly produced by exposure to a single injection of methylazoxymethanol acetate (MAM Ac) given to the pregnant jill on gestation day 32. Ten lissencephalic and 11 normal ferrets were chronically implanted with 14 cortical stainless steel electrodes. EEG records were sampled from various stages of the sleep/awake cycle. Six of each group were subjected to electrical stimulation for seizure threshold. Although the number of stimulations and the current intensity required to produce epileptiform afterdischarges (AD) and seizures were not different between the two groups, the lissencephalic ferrets had significantly longer AD and seizures, and a greater number of generalized seizures, indicating an enhanced seizure susceptibility. The EEG of the lissencephalic ferrets was characterized by increased slow wave activity within the low theta band range, extreme spindle activity, focal or multifocal slow and sharp waves, spikes, or spike and slow wave complexes. The differences in the EEG were more pronounced during drowsiness and sleep stages. The brains of all of the treated animals were lissencephalic and hydrocephalic, and weighed significantly less than those of the normals. The cerebral cortex was thin and flattened, with the parieto-occipital region most severely affected. Heterotopic foci were found in the cerebellum as well as in the cerebral cortex. Abnormalities in the configuration of the cerebellar folia were also seen. Comparison between the electrophysiological and neuropathological data suggests that the extent of the extreme spindle activity, and longer AD and seizure duration depended on the degree of cerebellar dysplasia, whereas the EEG focal abnormalities were related to lesions in the cerebral hemispheres.

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

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