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

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.

Consequences of recurrent seizures during early brain development

It is well documented that prolonged seizures (status epilepticus) can cause neuronal injury and result in synaptic reorganization in certain brain regions. However, the effect of recurrent, relatively short seizures in young animals on subsequent brain development is not known. To study the consequences of recurrent seizures on the developing brain, we subjected immature rats to a total of 50 flurothyl-induced seizures from postnatal day 11 until day 23. Immunohistochemistry for c-fos was performed to characterize the pattern of neuronal activation following the seizures. Cell counting of dentate granule cells, CA3, CA1, and hilar neurons, using unbiased stereological methods, and the silver impregnation method were used to evaluate neuronal death following the recurrent seizures. Timm and Golgi staining were performed four weeks after the 50th seizure to evaluate the effects of recurrent seizures on synaptic organization. Our results show that recurrent flurothyl-induced seizures progressively increased excitability of the brain, as revealed by a dramatic increase in the extent and intensity of c-fos immunostaining. While no cell loss was detected in the hippocampus with either Cresyl Violet or silver stains, animals experiencing multiple daily seizures developed increased mossy fiber sprouting in both the supragranular layer of the dentate gyrus and the infrapyramidale layer of the CA3 region. Golgi staining confirmed that there was an increase in mossy fibers in the pyramidal cell layer. Our results suggest that serial recurrent seizures in the immature brain can lead to significant changes in mossy fiber distribution even though the seizures do not cause significant hippocampal cell loss.

Mossy fiber sprouting after recurrent seizures during early development in rats

In some children, epilepsy is a catastrophic condition, leading to significant intellectual and behavioral impairment, but little is known about the consequences of recurrent seizures during development. In the present study, we evaluated the effects of 15 daily pentylenetetrazol-induced convulsions in immature rats beginning at postnatal day (P) 1, 10, or 60. In addition, we subjected another group of P10 rats to twice daily seizures for 15 days. Both supragranular and terminal sprouting in the CA3 hippocampal subfield was assessed in Timm-stained sections by using a rating scale and density measurements. Prominent sprouting was seen in the CA3 stratum pyramidale layer in all rats having 15 daily seizures, regardless of the age when seizures began. Based on Timm staining in control P10, P20, and P30 rats, the terminal sprouting in CA3 appears to be new growth of axons and synapses as opposed to a failure of normal regression of synapses. In addition to CA3 terminal sprouting, rats having twice daily seizures had sprouting noted in the dentate supragranular layer, predominately in the inferior blade of the dentate, and had a decreased seizure threshold when compared with controls. Cell counting of dentate granule cells, CA3, CA1, and hilar neurons, with unbiased stereological methods demonstrated no differences from controls in rats with daily seizures beginning at P1 or P10, whereas adult rats with daily seizures had a significant decrease in CA1 neurons. Rats that received twice daily seizures on P10-P25 had an increase in dentate granule cells. This study demonstrates that, like the mature brain, immature animals have neuronal reorganization after recurrent seizures, with mossy fiber sprouting in both the CA3 subfield and supragranular region. In the immature brain, repetitive seizures also result in granule cell neurogenesis without loss of principal neurons. Although the relationship between these morphological changes after seizures during development and subsequent cognitive impairment is not yet clear, our findings indicate that during development recurrent seizures can result in significant alterations in cell number and axonal growth.

Consequences of neonatal seizures in the rat: morphological and behavioral effects

Whereas neonatal seizures are a predictor of adverse neurological outcome, there is controversy regarding whether seizures simply reflect an underlying brain injury or can cause damage. We subjected neonatal rats to a series of 25 brief flurothyl-induced seizures. Once mature the rats were compared with control littermates for spatial learning and activity level. Short-term effects of recurrent seizures on hippocampal excitation were assessed by using the intact hippocampus formation preparation and long-term effects by assessing seizure threshold. Brains were analyzed for neuronal loss, sprouting of granule cell axons (mossy fibers), and neurogenesis. Compared with controls, rats subjected to neonatal seizures had impaired learning and decreased activity levels. There were no differences in paired-pulse excitation or inhibition or duration of afterdischarges in the intact hippocampal preparation. However, when studied as adults, rats with recurrent flurothyl seizures had a significantly lower seizure threshold to pentylenetetrazol than controls. Rats with recurrent seizures had greater numbers of dentate granule cells and more newly formed granule cells than the controls. Rats with recurrent seizures also had sprouting of mossy fibers in CA3 and the supragranular region. Recurrent brief seizures during the neonatal period have long-term detrimental effects on behavior, seizure susceptibility, and brain development.

Synaptic reorganization following kainic acid-induced seizures during development

Prolonged seizures in the adult brain causes neuronal loss in the hippocampus and aberrant growth (sprouting) of granule cell axons (mossy fibers) in the supragranular zone of the fascia dentata and stratum infrapyramidale of CA3. There is considerable evidence that these changes in neuronal growth following seizures are age related, with younger animals having fewer reactive changes following prolonged seizures than older animals. However, there is little information available regarding the age at which seizures in the developing brain result in alterations in axonal growth and synapse formation. In this study, we evaluated the effects of kainic acid (KA)-induced seizures during development on synaptic reorganization using the expression of growth-associated protein-43 (GAP-43), a marker for synaptogenesis and Timm stain which detects the presence of zinc in granule cell axons. Age specific doses of KA were used to induce seizures of similar intensity at various ages (postnatal days (P) 12, 21, 25, 35, 45, 60) in Sprague-Dawley rats. Up to the age of P25, there were no differences in either Timm or GAP-43 staining between animals with KA seizures and controls. In P25 and older KA-treated rats, Timm staining was found in the supragranular layer of the dentate gyrus. This staining increased with age at the time of KA injection. Seizures in adult (P60), but not younger rats also resulted in increased staining in the suprapyramidal layer of the CA3 subfields. Changes in GAP-43 were delayed compared to the Timm staining with no differences between KA-treated animals and controls until P35 when a band of GAP-43 immunostaining appeared in the supragranular inner molecular layer, progressively increasing in intensity and thickness with time. This study demonstrates that seizure-induced reactive synaptogenesis is age-related. Since both Timm and GAP-43 reflect different aspects of reactive synaptogenesis, used in combination these methods provide useful information about the structural changes following seizures during development.

The impaired long-term potentiation in the CA1 field of the hippocampus of cognitive deficient microencephalic rats is restored by D-serine

Rat embryos exposed on gestational day 15 to methyl-azoxymethanol acetate develop a microencephaly characterized primarily by a hypoplasia of the neocortex and CA fields of the hippocampus that in adulthood is associated with disturbances in learning. In brain slices prepared from microencephalic rats, we have examined the field excitatory postsynaptic potentials and population spike in the CA1 field of the hippocampus evoked by stimulation of the stratum radiatum. These parameters did not differ from those obtained in slices from control rats. High frequency stimulation of the stratum radiatum afferent fibres, which readily induced long-term potentiation of the field excitatory postsynaptic potentials and population spike in the CA1 field of the hippocampus of control rats, failed to induce long-term potentiation in that of microencephalic rats. High frequency stimulation of the perforant path readily elicited long-term potentiation in the dentate gyrus of both control and microencephalic rats. Picrotoxin had no apparent effect on field excitatory postsynaptic potentials and population spike in the CA1 field of the microencephalic rats, indicating that little GABAergic inhibition was present in slices from these rats. D-2-Amino-phosphonovalerate suppressed the field potentials in slices from microencephalic rats by more than 50%, suggesting that N-methyl-D-aspartate receptors contributed markedly to the synaptic responses evoked by single stimuli. D-Serine, but not picrotoxin, restored long-term potentiation in the CA1 field of the microencephalic rats. The D-serine effect was prevented by pretreating the slices with either 7-chloro-kynurenate or D-2-amino-phosphonovalerate. The failure to induce long-term potentiation, if also found in vivo, may be among the factors related to the learning deficits displayed by these 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.

A morphometric analysis of trimethyltin-induced change in rat brain using the Timm technique

Rats were given a single gavage of trimethyltin chloride (TMT) providing a dose of 0, 4.3, or 6.7 mg/kg of alkyltin. Gross changes in brain structures were quantified and analyzed statistically. Behavioral and functional measures were taken to verify efficacy of TMT dose. The high dose produced transient weight loss and seizures. In the fourth week after gavage, the high dose produced hyperactivity in the residential maze and activity wheel. High and low TMT doses decreased auditory startle responsiveness. Estrus cycle was normal in all groups. Brains were sectioned and stained with the Timm stain which delimited subregions of hippocampus and connected structures and also revealed mossy fibers. Linear and areal measures were made at three positions along the septotemporal axis of Ammon's horn. The low dose produced reductions in size in a few isolated subareas of the brain. The high dose produced, at the three planes studied, extensive (15-40%) loss of tissue in Ammon's horn and structures to which Ammon's horn is interconnected--subiculum, entorhinal cortex, dentate gyrus, hilus, CA3, and CA1 region. Neocortex and caudate-putamen were unaffected. These findings suggest that a single TMT gavage may disrupt brain structures important to linking neocortex with subcortex via structures in the hippocampal region.

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

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

When should we expect prenatally damaged human brains to have abnormal neuronal connections?

Abnormal function in prenatally damaged brains may occur as the result of loss of neuronal an/or glial elements, abnormal morphology of neurons, or abnormal and inappropriate connections. By abnormal connection I mean projections from a given nucleus to a nucleus or cortical region in the brain which does not normally receive such a projection. In the present paper it is proposed that focal brain damage, i.e. that damage involving circumscribed regions of the developing brain on one side of the body, is more likely to induce abnormal connections, than diffuse brain damage, affecting most of the brain bilaterally. This proposition is based on the available evidence from animal studies, albeit scanty, and is presented as a working hypothesis for future studies of human congenital brain damage.

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.

The organization and development of the hippocampal mossy fiber system

The anatomical organization and development of the hippocampal mossy fiber system has been reviewed with special reference to its organization in the common laboratory rat. The mossy fibers originate from the granule cells of the dentate granular layer and the few granule cells found scattered in the dentate molecular layer and hilus. Via a complex system of collaterals the mossy fibers terminate on several types of neurons in the hilus, e.g. the basket cells and the mossy cells. Upon leaving the hilus to pass into Ammon's horn, the mossy fibers converge to form a distinct band of fibers that terminates on the proximal part of the apical and basal dendrites of the pyramidal and basket cells of the regio inferior. In some mammalian species the mossy fibers may continue into the adjacent part of the regio superior. Despite differences in the number of granule cells and pyramidal cells at different septotemporal levels this organization is relatively uniform along the septotemporal extent of the hippocampus. During development the mossy fibers grow out in a sequential manner that matches the pattern of neurogenesis and the aggregation of the cells of origin. From the level at which they originate, the fibers diverge along the septotemporal axis in such a way that the oldest granule cells have the most extensive projections. The adult topographic organization, which is already apparent at the earliest developmental stages, is thus formed in a stepwise fashion. It is concluded that the organization of the hippocampal mossy fibers indicates that neuronal specificity should not be explained by cellular recognition alone, but rather as the cumulated product of the preceding sequence of developmental events that include neurogenesis, migration, aggregation and directed axonal outgrowth.

A light and electron microscopic analysis of the mossy fibers of the rat dentate gyrus

The axon collaterals of dentate granule cells have been analyzed with the aid of a computerized microscope, following intracellular injections of horseradish peroxidase in hippocampal slice preparations. The axon of each granule cell gives rise to approximately seven primary collaterals; these collaterals usually divide into secondary and tertiary branches, which form an extensive plexus within the hilar region of the dentate gyrus. Individual axon collaterals vary greatly in length, but most have been found to be between 100 and 300 microns long. On average, the summed lengths of the collaterals (exclusive of the parent mossy fiber) are approximately 2,300 microns. Except for an occasional collateral that is given off by a mossy fiber in the proximal part of field CA3 of the hippocampus, the collaterals of the granule cell axons are confined to the hilar region; they are rarely seen in the granule cell layer itself and have never been observed in the molecular layer. In the longitudinal dimension of the dentate gyrus, most of the collaterals are contained within a zone about 400 microns wide. The distribution of the collaterals within the hilar region is correlated with the location of the granule cell body. Those that arise from cells near the tip of the suprapyramidal blade tend to be confined to the region above field CA3; those from cells nearer the crest and from the infrapyramidal blade ramify widely throughout the hilus. Two types of varicosities are present on the collaterals. Numerous small (approximately 2 microns), round varicosities are distributed unevenly along the collaterals; in electron micrographs these varicosities can be seen to make asymmetric synaptic contacts with dendritic shafts. On average, each granule cell collateral plexus has about 160 of these varicosities. The second type of varicosity is irregular in shape and ranges from 2 to 4 microns in diameter; there is usually only one such varicosity per collateral. In all respects except size, these varicosities resemble the expansions found on the parent mossy fibers. Mossy fiber trajectories in the proximal part of field CA3 were studied after extracellular injections of HRP into localized regions of the granule cell layer. Granule cells at different locations around the blade send their mossy fibers to different depths within the pyramidal cell layer in the proximal part of field CA3. However, further distally, mossy fibers from all parts of the granule cell layer contribute to the suprapyramidal bundle that occupies the stratum lucidum.

Dendritic arbors and dendritic excrescences of abnormally positioned neurons in area CA3c of mice carrying the mutation "hippocampal lamination defect"

BALB/cJ and BALB/cByJ mice are homozygous for the autosomal gene "hippocampal lamination defect" (provisional gene symbol: Hld) which produces an abnormality in the lamination of the pyramidal cell layer of area CA3c of the hippocampus such that early-generated neurons are superficial and late-generated neurons are deep. Other inbred strains of mice are wild-type (+/+) at the Hld locus and do not have this inversion in cell position in area CA3c. The Golgi method was used to analyze the dendritic arbors of the abnormally positioned pyramidal cells and to compare the distribution of dendritic excrescences (i.e., the termination sites of the mossy fibers) in +/+ and Hld/Hld mice. It was found that in +/+ mice the late-generated pyramidal cells (whose cell bodies are positioned just below the suprapyramidal mossy fiber layer) have one set of dendritic excrescences on their apical dendrites as they extend through the suprapyramidal mossy fiber layer and a second set on their basal dendrites as they pass through the infrapyramidal mossy fiber layer. In contrast, in Hld/Hld mice the late-generated pyramidal cells (whose cell bodies are abnormally positioned just below the intrapyramidal mossy fiber layer) have two sets of dendritic excrescences on their apical dendrites, as they pass through the intrapyramidal and suprapyramidal mossy fiber layers, and none on their basal dendrites. In addition, in the vicinity of the apparent point of contact of the intrapyramidal mossy fibers, the apical dendrites of some of the abnormally positioned pyramidal cells have several fine-caliber branches.(ABSTRACT TRUNCATED AT 250 WORDS)

Extensive target cell loss during development results in mossy fibers in the regio superior (CA1) of the rat hippocampal formation

The axons of dentate granule cells (mossy fibers) have been reported to appear in the regio superior (CA1) of the rat hippocampal formation following destruction of the pyramidal cells in the regio inferior (CA3). We undertook the present experiments to confirm this finding and to determine the requirements for this dramatic neuronal rearrangement. We found that extensive (greater than 80%) loss of CA3 cells, as well as the presence of surviving CA1 neurons within a narrow period of development (postnatal days 3-5) is necessary, however apparently not sufficient, for the appearance of CA1 mossy fibers. That the absence of normal target cells during a restricted period of mossy fiber development will lead to their association with novel targets suggests that much of the specificity of this developing connection depends on the presence of normal targets during a critical period.

Effects of alcohol exposure during different periods of development: changes in hippocampal mossy fibers

Exposure to 10-12 g/kg/day of alcohol either during days 1-10 or 11-21 of gestation had no detectable effect on hippocampal mossy fiber development. Exposing artificially reared rat pups to 7.0-7.5 g/kg/day of alcohol during days 1-10 postpartum dramatically altered the organization of the Timm-stained mossy fiber terminal field when the animals were examined as adults, suggesting that alcohol exposure during a period equivalent to the human third trimester is more deleterious to brain development than exposure during periods equivalent to either the first or second trimesters.

The effect of in utero ethanol exposure on hippocampal mossy fibers: an HRP study

Adult rats, exposed to ethanol in utero exhibit aberrant mossy fiber-like Timm staining in the distal infrapyramidal region of the hippocampus at midtemporal levels. The present study utilized the anterograde transport of HRP to verify that the aberrant pattern of Timm staining represented a terminal field of the dentate granule cells. One or more HRP-labeled mossy fiber bundles were shown to cross to the infrapyramidal side of the pyramidal cell layer primarily, but not exclusively, at the same septotemporal level where the aberrant terminal field was located.

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

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