Effects of prenatal irradiation on the development of cerebral cortex and corpus callosum of the mouse

A multidisciplinary approach unravels early and persistent effects of X-ray exposure at the onset of prenatal neurogenesis

Background

In humans, in utero exposure to ionising radiation results in an increased prevalence of neurological aberrations, such as small head size, mental retardation and decreased IQ levels. Yet, the association between early damaging events and long-term neuronal anomalies remains largely elusive.

Methods

Mice were exposed to different X-ray doses, ranging between 0.0 and 1.0 Gy, at embryonic days (E) 10, 11 or 12 and subjected to behavioural tests at 12 weeks of age. Underlying mechanisms of irradiation at E11 were further unravelled using magnetic resonance imaging (MRI) and spectroscopy, diffusion tensor imaging, gene expression profiling, histology and immunohistochemistry.

Results

Irradiation at the onset of neurogenesis elicited behavioural changes in young adult mice, dependent on the timing of exposure. As locomotor behaviour and hippocampal-dependent spatial learning and memory were most particularly affected after irradiation at E11 with 1.0 Gy, this condition was used for further mechanistic analyses, focusing on the cerebral cortex and hippocampus. A classical p53-mediated apoptotic response was found shortly after exposure. Strikingly, in the neocortex, the majority of apoptotic and microglial cells were residing in the outer layer at 24 h after irradiation, suggesting cell death occurrence in differentiating neurons rather than proliferating cells. Furthermore, total brain volume, cortical thickness and ventricle size were decreased in the irradiated embryos. At 40 weeks of age, MRI showed that the ventricles were enlarged whereas N-acetyl aspartate concentrations and functional anisotropy were reduced in the cortex of the irradiated animals, indicating a decrease in neuronal cell number and persistent neuroinflammation. Finally, in the hippocampus, we revealed a reduction in general neurogenic proliferation and in the amount of Sox2-positive precursors after radiation exposure, although only at a juvenile age.

Conclusions

Our findings provide evidence for a radiation-induced disruption of mouse brain development, resulting in behavioural differences. We propose that alterations in cortical morphology and juvenile hippocampal neurogenesis might both contribute to the observed aberrant behaviour. Furthermore, our results challenge the generally assumed view of a higher radiosensitivity in dividing cells. Overall, this study offers new insights into irradiation-dependent effects in the embryonic brain, of relevance for the neurodevelopmental and radiobiological field.

Electronic supplementary material

The online version of this article (doi:10.1186/1866-1955-7-3) contains supplementary material, which is available to authorized users.

A mouse model of ATR-Seckel shows embryonic replicative stress and accelerated aging

Although DNA damage is considered a driving force for aging, the nature of the damage that arises endogenously remains unclear. Replicative stress, a source of endogenous DNA damage, is prevented primarily by the ATR kinase. We have developed a mouse model of Seckel syndrome characterized by a severe deficiency in ATR. Seckel mice show high levels of replicative stress during embryogenesis, when proliferation is widespread, but this is reduced to marginal amounts in postnatal life. In spite of this decrease, adult Seckel mice show accelerated aging, which is further aggravated in the absence of p53. Together, these results support a model whereby replicative stress, particularly in utero, contributes to the onset of aging in postnatal life, and this is balanced by the replicative stress-limiting role of the checkpoint proteins ATR and p53.

Brain Atrophy after Foetal Exposure to Very Low Doses of Ionizing Radiation

Acute, high dose-rate, exposure of the rat embryo on day 15 post-conception (PC) causes a reduction of brain weight in adult life that is proportional to the dose received. Doses as low as 10 mGy of 600 keV neutrons, from a Van de Graaff accelerator, or 100 mGy of 250 kV X-rays are capable of eliciting a significant effect. The relative biological effectiveness for acute neutron exposure compared with 250 kV X-rays was 3.5. A brain weight reduction was also observed after gamma-ray exposures protracted over 4 or 6 days, during cerebral corticogenesis. The dose-rate reduction factor was only 1.5 for exposure from days 12 to 16 PC and 3.3 for exposure from days 14 to 20 PC. In relation with the decrease in brain weight, the cingulum bundle, a myelinated structure associated with the corpus callosum, displayed a significant reduction in size. The implications of these observations for human exposures are discussed.

Cytoarchitecture and transcriptional profiles of neocortical malformations in inbred mice

Malformations of neocortical development are associated with cognitive dysfunction and increased susceptibility to epileptogenesis. Rodent models are widely used to study neocortical malformations and have revealed important genetic and environmental mechanisms that contribute to neocortical development. Interestingly, several inbred mice strains commonly used in behavioral, anatomical, and/or physiological studies display neocortical malformations. In the present report we examine the cytoarchitecture and myeloarchitecture of the neocortex of 11 inbred mouse strains and identified malformations of cortical development, including molecular layer heterotopia, in all but one strain. We used in silico methods to confirm our observations and determined the transcriptional profiles of cells found within heterotopia. These data indicate cellular and transcriptional diversity present in cells in malformations. Furthermore, the presence of dysplasia in nearly every inbred strain examined suggests that malformations of neocortical development are a common feature in the neocortex of inbred mice.

Unraveling the fundamental molecular mechanisms of morphological and cognitive defects in the irradiated brain

Prenatal radiation exposure may have serious consequences for normal brain development. Results of epidemiological studies clearly pointed towards an increased risk of mental retardation in children of the surviving women of the Hiroshima/Nagasaki atomic bombing when in utero exposure had occurred between weeks 8 and 15 of pregnancy or, at a lower extend between weeks 15 and 25. The high sensitivity of the developing brain, in comparison to the adult brain is related to its higher number of non-differentiated, dividing neural precursor cells. Exposure of the developing brain to ionizing radiation can lead to three main outcomes in the developing brain, depending on the radiation dose and the elapsed period after irradiation. A first event occurs early after irradiation and triggers disturbances in cell proliferation, migration, differentiation, and cell death. A second event involves the generation of morphological abnormalities in the developing brain, if the radiation dose is sufficient. A third event involves cognitive dysfunctions that are a direct consequence from a disturbance in regional brain formation. The latter results from exposure to low doses and is usually only observed in the later period of development. In order to understand the mechanisms of radiation-induced cognitive dysfunctions, it is important to track back the underlying changes in specific molecular pathways. In this review, we present the possible relationships within and between molecular pathways potentially involved in cognitive dysfunctions induced by ionizing radiation in the developing brain.

Callosal agenesis and absence of primary visual cortex induced by prenatal X rays impair navigation's strategy and learning in tasks involving visuo-spatial working but not reference memory in mice

This study was designed for the identification of possible and distinct abilities for behavioral recovery after prenatal cerebral damage. We adopted an interesting tool for promotion of cell's death. Due to the fact that neuroblastic cells and early postmitotic neurons on the beginning of differentiation are particularly sensible for the promotion of apoptosis, we used a low whole-body dose of X radiation on pregnant female mice on E16 (sixteenth gestational day) to promote damage on specific cerebral areas of the progeny, given that the pattern of cerebral neurogenesis is not homogeneous. The morphological results were previously described by our team. Here we noticed that the recovery of behavioral functions after prenatal damage seems to be related to specific factors of local cortical circuitry organization. The deficits found on visual navigation and working memory contrast with the recovery of primary visual functions and also with reference memory, where the mice have a delay on acquisition of learning but get it. As a conclusion we reasoning that changes on laminar organization on frontal cortex as well as the inter hemispheric cortical integration through the corpus callosum could promote relatively fixed cognitive dysfunctions, as those observed on performances that require strategies for navigation (decision making) and working memory, with consequences also observed on the subsequent learning.

Increased lateralization in rotational side preference in male mice rendered acallosal by prenatal gamma irradiation

In order to test the hypothesis that the ontogenetic development of the corpus callosum is related to the establishment of behavioral laterality, the rotatory behavior in the free-swimming test was studied in male Swiss mice with callosal defects induced by exposure to gamma irradiations at the 16th embryonic day (total dose of 3 Gy). At adulthood, 43 irradiated and 56 non-irradiated mice were submitted to 3 sessions of the free-swimming rotatory test (diameter of the recipient=21 cm; session duration=5 min; inter-test interval=48 h). The number and direction of 30 degrees and 360 degrees turns were recorded. Animals were classified as side-consistent turners (to the right or to the left) when they did not change their preferred side of rotation in all three sessions and in both turning units. In general our results suggested that irradiated animals present more pronounced laterality than non-irradiated ones. In the irradiated group, the percentage of consistent turners was significantly higher than that of non-consistent turners. In the first session, the percentage of animals that presented strong turning preferences in the acallosal group was higher than in the normal group. In first session, the acallosal group presented a higher average number of turns to preferred side than the normal group. Taken together, our results constitute an endorsement to the hypothesis that the normal development of the corpus callosum is related to the establishment of cerebral laterality.

Multiple defects in the formation of rat cortical axonal pathways following prenatal X-ray irradiation

Prenatal X-ray irradiation is known to result in severe defects of neuronal migration and laminar formation in the cerebral cortex. We examined the formation of cortical afferent and efferent pathways in rats that had been exposed to X-ray irradiation (1.0 Gy) at embryonic day 14 (E14), by birthdating with bromodeoxyuridine (BrdU) and axonal labeling with 1-1'-dioctodecyl-3,3,3',3'- tetramethyl-indocarbocyanine perchlorate (DiI), in addition to immunohistochemical staining for various axonal markers including neurofilament, and cell adhesion molecules L1 and TAG-1. The results obtained were as follows. (i) The neuroepithelium formed germinal rosettes and concavities in the cortical anlage from 2 days after irradiation. Neurons generated in the neuroepithelium accumulated to form subcortical heterotopia and obstructed pathway formation in the intermediate zone, resulting in an aberrant trajectory of TAG-1-immunoreactive cortical efferent axons. (ii) In rats exposed to X-ray irradiation at E14, cystic cavities were formed in the cortex-striatum boundary region between E15 and E17, probably because of delayed cell death of neurons generated at E14. These cavities transiently interrupted both cortical afferent (L1-positive) and efferent (TAG-1-positive) axons. (iii) X-ray irradiation at E14 partially destroyed subplate neurons (transient targets of thalamic afferent axons) and disturbed the arrangement of the subplate layer. This resulted in a misrouting of neurofilament- and L1-immunoreactive thalamocortical axons that obliquely traversed the cortical plate to run up to the superficial layer. The present study demonstrates for the first time that X-ray irradiation during initial cortical development causes multiple defects in the formation of cortical afferent and efferent pathways.

Effects of callosal agenesis on rotational side preference of BALB/cCF mice in the free swimming test

In order to test the hypothesis that the ontogenetic development of the corpus callosum is related to the establishment of behavioral laterality, the rotatory behavior in the free swimming test was studied in male mice of the BALB/cCF strain, in which approximately 20% of the animals present total or partial callosal agenesis. All animals were submitted to three sessions of the free-swimming rotatory test in three different sessions (diameter of the recipient = 21 cm; session duration = 5 min; inter-test interval = 48 h). The number and direction of the 30 and 360 degrees turns were recorded. Animals were classified as side-consistent turners (to the right or to the left) when they did not change their preferred side of rotation in all three sessions and in both turning units. In general our results suggested that acallosal animals present more pronounced laterality than normal ones. In the acallosal group, the percentage of consistent turners was significantly higher than that of non-consistent turners. The percentage of animals that presented strong turning preferences in the acallosal group was higher than in the normal group. In first session, the acallosal group presented a higher average number of turns to preferred side than the normal group. Taken together, our results constitute an endorsement to the hypothesis that the normal development of the corpus callosum is related to the establishment of cerebral laterality.

Neonatal transection of the corpus callosum affects paw preference lateralization of adult Swiss mice

In the present work, the hypothesis that the ontogenetic development of the corpus callosum (CC) affects the establishment of behavioral lateralization was tested by studying paw preference performance in adult Swiss mice that were subjected to mid-sagittal transection of the CC on the first postnatal day. Magnitude and direction of laterality were evaluated independently. No significant differences between groups were found for the magnitude of paw preference. On the other hand, the transected group presented a significant populational bias favoring the left paw that was not present in the control groups. These results lend support to the hypothesis that the development of the CC plays a role in the establishment of the normal pattern of behavioral lateralization.

Time course of the effects of prenatal gamma irradiation on the dorsal lateral geniculate nucleus of Swiss mice

A previous study reported that adult mice irradiated at the 16th embryonic day present a severe neuronal number reduction in the dorsal lateral geniculate thalamic nucleus. In the present study, we investigated the time course of the effects of prenatal irradiation on this thalamic nucleus. One day after irradiation, a great number of pyknotic figures were seen mainly in the cerebral proliferative zones. In the geniculate nucleus, only scattered pyknotic figures were identified. On the first week after birth, the geniculate nucleus presented frequent pyknotic figures. From five days after birth onwards, a severe shrinkage of the occipital cortex and a great reduction in the geniculate nucleus neuronal number were found. On the second week after birth this neuronal number reduction reached as high as 75%. At each postnatal analyzed age, severe volumetric geniculate nucleus shrinkage was combined to non-significant neuronal density variations. The presence of few pyknotic figures in the geniculate nucleus one day after irradiation and its delayed neuronal loss indicate an indirect effect of irradiation. We suggest that the effect upon the geniculate nucleus is secondary to the damage of the occipital cortex. A possible interpretation for thalamic neuronal loss is that geniculate neurons fail to establish cortical arbors after major target loss. In this case, the loss of trophic support should also be considered.

Effects of prenatal ionizing irradiation on the development of the ganglion cell layer of the mouse retina

Prenatal exposure to ionizing irradiation has been shown to be an effective method to eliminate selectively certain neuronal population. This investigation studied the effects on the ganglion cell layer of the retinae of adult mice exposed to a gamma source (total dose=3 Gy) at 16 days gestation. There was a significant reduction in the total number of neurons (displaced amacrine+ganglion cells) in the ganglion cell layer (33%) that was mainly caused by a pronounced loss (59%) of displaced amacrine cells. The diameters of the surviving retinal ganglion cells were consistently larger than those of the controls. Prenatal irradiation is the first experimental approach that partially eliminates displaced amacrine cells. It is suggested that the morphogenesis of retinal ganglion cells may be affected by displaced amacrine cells.

Unilateral induced neocortical malformation and the formation of ipsilateral and contralateral barrel fields

Freezing lesions to the developing cortical plate of rodents results in a focal malformation resembling human 4-layered microgyria, and this malformation has been shown to result in local and widespread disruptions of neuronal architecture, connectivity, and physiology. Because we had previously demonstrated that microgyria caused disruptions in callosal connections, we hypothesized that freeze lesions to the postero-medial barrel sub-field (PMBSF) in one hemisphere would affect the organization of this barrel field contralaterally. We placed freeze lesions in the presumptive PMBSF of neonatal rats and, in adulthood, assessed the architecture of the ipsilateral and contralateral barrel fields. Malformations in the PMBSF resulted in a substantial decrease in the number of barrels as identified by cytochrome oxidase activity. More importantly, we found an increase in the total area of the contralateral PMBSF, although there was no difference in individual barrel cross-sectional areas, indicating an increase in the area of inter-barrel septae. This increase in the septal area of the contralateral PMBSF is consistent with changes in callosal and/or thalamic connectivity in the contralateral hemisphere. These results are another example of both local and widespread disruption of connectional architecture following induction of focal microgyria.

Depletion of cortical target induced by prenatal ionizing irradiation: effects on the lateral geniculate nucleus and on the retinofugal pathways

Studies using neonatal surgical lesions to reduce the target area of the retina have supported the idea that developing axons show only a limited specificity in their targeting. This investigation tested whether retinogeniculate axons adjust for partial target depletion by repositioning of axons. We used adult Swiss mice exposed to gamma rays at the time when layer IV cells are generated in the ventricular zone (16 days of gestation). Nissl-stained brain sections were used for histological analyses in thalamus and cortex. Retinal ganglion cells were backfilled from the optic tract with horseradish peroxidase. Intraocular injections of horseradish peroxidase were used to study the retinal projections. In the posterior cortex there was a nearly complete absence of layer IV. The irradiated animals showed a 75% reduction of the dorsal lateral geniculate nucleus. The ventral division, superior colliculus, and other visually related nuclei were not affected. The loss in the ganglion cells (15.7%) was significant but clearly smaller than that observed in the dorsal lateral geniculate nucleus (75%). Therefore, the shrinkage of the dorsal lateral geniculate nucleus led to a reduction in the area available for retinal projections. Despite partial target loss, pattern of retinal projections did not differ from that of the controls. The effect on the dorsal lateral geniculate nucleus is discussed in the light of differences between prenatal and neonatal damage of the presumptive visual cortex. The absence of aberrant retinal projections suggests that repositioning of axons is not the first mechanism employed by retinal axons to match connections in numerically disparate populations.

Effects of prenatal gamma irradiation on the development of the corpus callosum of Swiss mice

The temporal sequence of events related to the effects of prenatal gamma irradiation on the development of the corpus callosum and cerebral cortex was studied in Swiss mice. Pregnant females on gestational day 16 were exposed to a 60Co source receiving total doses of 2 or 3 Gy. The offspring were analyzed at both prenatal and postnatal days. One day after irradiation, a great number of pyknotic figures was seen along the whole extension of the cerebral wall, especially in the proliferative zones. At perinatal ages, the thickness of the proliferative zones was reduced and the glial sling was never identified. From 5 days after birth onwards, we observed a severe shrinkage of layers II + III and IV. The majority of the irradiated mice were totally acallosal (particularly when the 3 Gy dose was used), but some animals presented callosal remnants. These remnants were identified above the ventral hippocampal commissure, except for two animals in which a larger callosal remnant extended from the columns of the fornix to the dorsal hippocampal commissure. The presence of callosal remnants in animals irradiated with 3 Gy was dependent on the age at which the animals were analyzed since remnants were observed in some animals analyzed at perinatal ages, but never in older animals. Callosal defects can be explained at least by three factors: (1) Death of a great part of callosal neurons located at layer III. (2) Postnatal axonal elimination. (3) Absence of the glial sling. The callosal agenesis in the absence of the glial sling indicates that this structure may play a crucial role in guiding callosal axons. However, the presence of callosal remnants indicates that surviving callosal axons can use structures other than the sling to cross the midplane. Our data indicate that axons of the middle portion of the callosum can cross the midplane using the ventral hippocampal commissure as a guide. Additionally, the dorsal hippocampal commissure may play a role in directing axons of the posterior part of the corpus callosum.

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.

Glutamate acting at NMDA receptors stimulates embryonic cortical neuronal migration

During cortical development, embryonic neurons migrate from germinal zones near the ventricle into the cortical plate, where they organize into layers. Mechanisms that direct neuronal migration may include molecules that act as chemoattractants. In rats, GABA, which localizes near the target destination for migrating cortical neurons, stimulates embryonic neuronal migration in vitro. In mice, glutamate is highly localized near the target destinations for migrating cortical neurons. Glutamate-induced migration of murine embryonic cortical cells was evaluated in cell dissociates and cortical slice cultures. In dissociates, the chemotropic effects of glutamate were 10-fold greater than the effects of GABA, demonstrating that for murine cortical cells, glutamate is a more potent chemoattractant than GABA. Thus, cortical chemoattractants appear to differ between species. Micromolar glutamate stimulated neuronal chemotaxis that was mimicked by microM NMDA but not by other ionotropic glutamate receptor agonists (AMPA, kainate, quisqualate). Responding cells were primarily derived from immature cortical regions [ventricular zone (vz)/subventricular zone (svz)]. Bromodeoxyuridine (BrdU) pulse labeling of cortical slices cultured in NMDA antagonists (microM MK801 or APV) revealed that antagonist exposure blocked the migration of BrdU-positive cells from the vz/svz into the cortical plate. PCR confirmed the presence of NMDA receptor expression in vz/svz cells, whereas electrophysiology and Ca2+ imaging demonstrated that vz/svz cells exhibited physiological responses to NMDA. These studies indicate that, in mice, glutamate may serve as a chemoattractant for neurons in the developing cortex, signaling cells to migrate into the cortical plate via NMDA receptor activation.

Morphological brain asymmetries in male mice with callosal defects due to prenatal gamma irradiation

We have previously suggested a relationship between the development of the corpus callosum and the direction of cerebral asymmetries (Schmidt and Caparelli-Daquer, 1989; Schmidt et al., 1991). Here we report a study on morphological brain asymmetries using a distinct experimental approach. At embryonic day 16, mice were exposed to a gamma source, receiving a total dose of 2 Gy. At adulthood 32 irradiated and 28 normal male Swiss mice were evaluated for individual and populational interhemispheric asymmetries of weight, dorsal area, and neocortical volume. All irradiated mice showed callosal defects ranging from total absence to a partial reduction of the midsagittal callosal area. The normal and irradiated groups displayed a pronounced individual asymmetry in all measurements. In contrast, populational asymmetry could be identified only in the normal group. These results are consistent with our previous data in acallosal mutant mice and support the hypothesis that the corpus callosum may play a role in directing interhemispheric morphological brain asymmetries.

In utero irradiation of rats as a model of human cerebrocortical dysgenesis: a review

Certain developmental abnormalities of the cerebral cortex are closely associated with epilepsy in humans. Exposure of fetal rats to external gamma-irradiation produces diffuse cortical dysplasia and neuronal heterotopia. These abnormalities are the result of radiation-induced cell death coupled with continued cortical development in an altered cellular environment. In vivo electroencephalography studies in these animals have revealed an increased propensity for electrographic seizures in the presence of the sedating agents, acepromazine and xylazine. In vitro neocortical slices containing dysplastic cortex demonstrate enhanced excitability, as compared to control neocortex, when inhibition that is mediated by the A-type gamma-amino butyric acid receptor is blocked with bicuculline methiodide. In utero irradiation of rats produces structural changes that mimic some aspects of cerebral dysgenesis in humans and results in physiologic changes that increase the animals' propensity for seizures. Similarities and differences between the animal model and the human syndromes are discussed.

Prenatal events and genetic factors in epileptic patients with neuronal migration disorders

Because disorders of neuronal migration to cerebral cortex in humans are believed to occur in the first half of gestation, prenatal events or genetic factors are suspected to have a pathogenetic role. We evaluated this by comparing the frequency of potentially harmful prenatal events and of genetic factors in a series of 40 patients (38 with epilepsy) with neuronal migration disorders (NMD) and in 40 epileptic controls, using a predetermined standardized questionnaire to minimize interviewer bias. Potentially harmful prenatal events (significant maternal physical trauma, ingestion of medications, exposure to roentgenograms, infections, uterine or metabolic abnormalities) were reported in the pregnancy histories of 58% of patients with NMD but in only 15% of epileptic controls (p = 0.0002). In contrast, peri- and postnatal potentially relevant etiologic factors were reported in the histories of only 22% of patients with NMD but in 50% of the epileptic controls (p = 0.01). Genetic factors (a family history of epilepsy, mental retardation, or congenital malformations of the CNS) were noted in 13 and 20% of the families, respectively. Stillbirths occurred only in the group with NMD, accounting for 3.06% of sibling pregnancies. The findings suggest that prenatal potentially harmful environmental events play a central role in the pathogenesis of NMD in humans.

Cortical axon trajectories and growth cone morphologies in fetuses of acallosal mouse strains

Hereditary absence of the corpus callosum (CC) provides an ideal experiment of nature for exploring mechanisms of axon guidance. In this study the prenatal development of CC axons in the acallosal mouse strains BALB/cWah1 and 129/ReJ or J was compared with normal hybrid mice by using the lipophilic dyes DiI and DiA. A few I/LnJ mice were also examined. The time of emergence and growth rate of CC axons from four cortical regions (frontal, parietal, temporal, occipital) were normal in acallosal strains. Their CC axons arrived at midplane on schedule but then often looped back to form the longitudinal Probst bundle. The frequency of formation of the Probst bundle was highest for axons from frontal cortex, which arrived at midplane first, and lowest for occipital axons, which arrived last. Once a few CC axons found a path to the other side via the hippocampal commissure, those that arrived later then crossed relatively normally. Some axons from the Probst bundle also managed to traverse midline in this manner. When no CC axons crossed, almost all of them entered the Probst bundle and eventually left it within a few hours to proceed in the ipsilateral white matter, never turning back toward midplane. Growth cones approaching midplane ipsilaterally and those that had crossed midline and entered contralateral white matter, as well as CC axons in the Probst bundle, expressed a normal range of size and complexity. These results demonstrate that the problem with callosal agenesis resides not in the cells of origin or the axons or growth cones themselves but in the substrates of axon guidance at the midsagittal plane.

The ontogenesis of the forebrain commissures and the determination of brain asymmetries

We have reviewed the organization and development of the interhemispheric projections through the forebrain commissures, especially those of the CC, in connection with the development of brain asymmetries. Analyzing the available data, we conclude that the developing CC plays an important role in the ontogenesis of brain asymmetries. We have extended a previous hypothesis that the rodent CC may exert a stabilizing effect over the unstable populational asymmetries of cortical size and shape, and that it participates in the developmental stabilization of lateralized motor behaviors.

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.

The effects of total and partial callosal agenesis on the development of paw preference performance in the BALB/cCF mouse

The relation between callosal defects and paw preference was investigated in 85 male mice of the BALB/cCF strain. Paw preference was also studied in 30 male Swiss mice. Directional laterality and magnitude of laterality devoid of directionality were evaluated independently. The study of the magnitude of paw preference showed that male BALB/cCF mice were more strongly lateralized than Swiss mice. There was no difference between BALB/cCF mice with callosal defects (abnormal group) and normal BALB/cCF mice. The analysis of directional laterality indicated a population tendency for left-paw use in BALB/cCF as compared to Swiss mice. Furthermore, the percentage of left-pawed animals in the abnormal group (78%) was significantly different from chance level, as opposed to an absence of such differences in the normal BALB/cCF and in Swiss mice. It was concluded that developmental disturbance of the corpus callosum is related to the appearance of a directional populational asymmetry in paw preference.

Genetic and developmental defects of the mouse corpus callosum

Among adult BALB mice fewer than 20% usually have a small or absent corpus callosum (CC) and inheritance is polygenic. In the fetus at the time when the CC normally forms, however, almost all BALB mice show a distinct bulge in the interhemispheric fissure and grossly retarded commissure formation, and inheritance appears to result from two autosomal loci, provided the overall maturity of fetuses is equated. Most fetuses recover from the early defect when the CC axons manage to cross over the hippocampal commissure, and thus there is developmental compensation for a genetic defect rather than arrested midline development. The pattern of interhemispheric connections when the adult CC is very small is topographically normal in most respects, despite the unusual paths of the axons. The proportion of mice which fail to recover completely can be doubled by certain features of the maternal environment, and the severity of defects in adults can also be exacerbated by new genetic mutations which create new BALB substrains. The behavioral consequences of absent CC in mice are not known, nor have electrophysiological patterns been examined. The mouse provides an important model for prenatal ontogeny and cortical organization in human CC agenesis, because these data are not readily available for the human condition.