The corpus callosum provides a massive transitory input to the visual cortex of cat and rat during early postnatal development

Spatio-temporal dynamics of neocortical presynaptic terminal development using multi-photon imaging of the corpus callosum in vivo

Within the developing central nervous system, the dynamics of synapse formation and elimination are insufficiently understood. It is ideal to study these processes in vivo, where neurons form synapses within appropriate behavioral and anatomical contexts. In vivo analysis is particularly important for long-range connections, since their development cannot be adequately studied in vitro. The corpus callosum (CC) represents a clinically-relevant long-range connection since several neurodevelopmental diseases involve CC defects. Here, we present a novel strategy for in vivo longitudinal and rapid time-lapse imaging of CC presynaptic terminal development. In postnatal mice, the time-course of CC presynaptic terminal formation and elimination was highly variable between axons or groups of axons. Young presynaptic terminals were remarkably dynamic - moving, dividing to generate more boutons, and merging to consolidate small terminals into large boutons. As synaptic networks matured, presynaptic mobility decreased. These rapid dynamics may be important for establishing initial synaptic contacts with postsynaptic partners, refining connectivity patterns or modifying synapse strength during development. Ultimately, this in vivo imaging approach will facilitate investigation of synapse development in other long-range connections and neurodevelopmental disease models.

Contralateral targeting of the corpus callosum in normal and pathological brain function

The corpus callosum connects the two cortical hemispheres of the mammalian brain and is susceptible to structural defects during development, which often result in significant neuropsychological dysfunction. To date, such individuals have been studied primarily with regards to the integrity of the callosal tract at the midline. However, the mechanisms regulating the contralateral targeting of the corpus callosum, after midline crossing has occurred, are less well understood. Recent evidence suggests that defects in contralateral targeting can occur in isolation from midline-tract malformations, and may have significant functional implications. We propose that contralateral targeting is a crucially important and relatively under-investigated event in callosal development, and that defects in this process may constitute an undiagnosed phenotype in several neurological disorders.

Alcohol exposure during the first two trimesters-equivalent alters the development of corpus callosum projection neurons in the rat

Children exposed prenatally to alcohol can display a variety of neural deficits, including an altered development of the corpus callosum (CC), the largest interhemispheric axon pathway in the brain. Furthermore, these children show functional abnormalities that are related to brain regions with significant numbers of CC connections. Little is known about how alcohol imparts influence on CC development, but one possible mechanism is by affecting the corpus callosum projection neurons (CCpn) directly. The purpose of this study was to quantify the effects of prenatal alcohol exposure on the number, size, and distribution of CCpn within the visual cortex. The visual cortex was selected specifically due to the many vision-related deficits noted in fetal alcohol exposed children and because the critical role of the CC in visual cortex development is well documented. Sprague-Dawley rat pups received one of four alcohol dosages during gestational days (G) 1-20, or reared as nutritional or untreated control animals. Each litter was categorized according to the peak blood alcohol concentration experienced. Pups were removed from each litter on days equivalent to G29, G36, G43, and G50, for histology and measurement. Callosal axons were labeled retrogradely to their CCpn using 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) and the CCpn were then examined using confocal laser scanning microscopy. Differences between alcohol-exposed and control animals were observed in CCpn cell body size, number, and location with the cortex. This was particularly true of animals exposed to high doses of alcohol. In addition, some trends of CCpn development were found to be unchanged as a result of prenatal alcohol exposure. The results demonstrate clear differences in the development of CCpn in the visual cortex between alcohol-exposed and control animals and suggest that this development is particularly affected in those animals exposed to high doses of alcohol.

GABAergic and pyramidal neurons of deep cortical layers directly receive and differently integrate callosal input

We studied the involvement of deep cortical layer neurons in processing callosal information in the rat. We observed with electron microscopy that both parvalbumin (PV)-labeled profiles and unlabeled dendritic spines of deep cortical layer neurons receive synapses from the contralateral hemisphere. Stimulation of callosal fibers elicited monosynaptic excitatory postsynaptic currents in both layer VI pyramidal neurons and gamma-aminobutyric acidergic (GABAergic) interneurons immunopositive for the vesicular GABA transporter and PV. Pyramidal cells had intrinsic electrophysiological properties and synaptic responses with slow kinetics and a robust N-metyhl-D-aspartate (NMDA) component. In contrast, GABAergic interneurons had intrinsic membrane properties and synaptic responses with faster kinetics and a less pronounced NMDA component. Consistent with these results, the temporal integration of callosal input was effective over a significantly longer time window in pyramidal neurons compared with GABAergic interneurons. Interestingly, callosal stimulation did not evoke feedforward inhibition in all GABAergic interneurons and in the majority of pyramidal neurons tested. Furthermore, retrogradely labeled layer VI pyramidal neurons of the contralateral cortex responded monosynaptically to callosal stimulation, suggesting interconnectivity between callosally projecting neurons. The data show that pyramidal neurons and GABAergic interneurons of deep cortical layers receive interhemispheric information directly and have properties supporting their distinct roles.

Developmental changes in the vascular network of the rat visual areas 17, 18 and 18a

The present study examines quantitatively the areal and the laminar fluctuations of the vascular network in the visual areas 17, 18 and 18a of the rat cerebral cortex, from postnatal day (P) 1 to P60. For this purpose, the detailed vascular networks of the visual areas, marked after transcardial perfusion of India ink, are analyzed with the use of an image analysis system in order to measure the total vascular density (VD) and the relative density of capillaries (CD), of medium (MD)- and large (LD)-sized vessels in combination with changes in the mean diameter of all three types of vessels. Comparative quantitative microscopy showed that both VD and CD do not exhibit significant interareal differences in the adult rat brain. However, while VD reaches adult values much earlier in area 18a (P21) than in areas 17 and 18 (P60), CD obtains adult values at P31 in areas 17 and 18a, but later (P60) in area 18. Maturation process of laminar VD, CD, MD and LD was not found to follow a simple (i.e. inside-out or mediolateral) sequence, and, in each cortical area, laminar fluctuations of vessels density revealed a complicated developmental pattern, which might be attributed to their changing structural and functional status. Developmental changes in the diameter of capillaries, examined in conjunction with concomitant changes of vascular and capillary density in each area, suggest the existence of angiogenesis in all three visual areas during the third postnatal week of age.

Integrin-mediated dendrite branch maintenance requires Abelson (Abl) family kinases

Dendrite arbor structure is a critical determinant of nervous system function that must be actively maintained throughout life, but the signaling pathways that regulate dendrite maintenance are essentially unknown. We report that the Abelson (Abl) and Abl-related gene (Arg) nonreceptor tyrosine kinases are required for maintenance of cortical dendrites in the mouse brain. arg-/- cortical dendrites initially develop normally and are indistinguishable from wild-type dendrites at postnatal day 21. Dendrite branches are not efficiently maintained in arg-/- neurons, leading to a reduction in dendrite arbor size by early adulthood. More severe dendrite loss is observed in abl-/-arg-/- neurons. Elevation of Arg kinase activity in primary cortical neurons promotes axon and dendrite branching. Activation of integrin receptors by adhesion to laminin-1 or Semaphorin 7A also promotes neurite branching in cortical neurons, but this response is absent in arg-/- neurons because of the reduced dynamic behavior of mutant neurite branches. These data suggest that integrin signaling through Abl and Arg support cortical dendrite branch maintenance by promoting dendrite branch dynamics in response to adhesive cues.

Corpus callosum and visual cortex of mice with deletion of the NMDA-NR1 receptor: I. Accelerated development of callosal projection neurons

Many pharmacological experiments show that the ionotropic receptor NMDA has both neurotrophic and neuroexcitotoxic effects. The neurotrophic function is manifested in many ways including acceleration of neuronal development, enhancement of neuronal migration, neuroprotection, blockage of apoptosis, prevention of aging and prematurity, as well as effects on synaptic plasticity and synaptogenesis. On the other hand, the neuroexcitotoxic function is manifested in its role in neurological and psychiatric diseases such as epilepsy, Parkinson's disease and schizophrenia. The present study explores the consequences of complete and partial absence of NMDA-NR1 receptors throughout development. Using DiI tracing in vitro, the development of corpus callosum projection neurons in transgenic mice with deletion of the NMDA-NR1 receptor was observed in visual cortex. Compared to littermate controls, the histogenesis and neuronal development of corpus callosum cells of origin was found to be accelerated in NR1-/- mice. That is, the corpus callosum projection neurons in NR1 knockout mice developed earlier and faster than in littermate heterozygous and wild-type mice. However, the corpus callosum projection neurons in NR1 heterozygous mice developed earlier and faster than in littermate wild-type mice. This suggests that NMDA-NR1 receptors are involved in sequencing and/or temporal regulation of neuronal development, and that there is a gene-dose effect. Studies from other laboratories suggest that the observed phenomenon of prematurity or accelerated development is a direct effect of altered expression of genes found in mice with deletion of the NMDA-NR1 receptor.

Second trimester prenatal alcohol exposure alters development of rat corpus callosum

Prenatal alcohol exposure produces many developmental defects of the central nervous system (CNS), such as in the corpus callosum (CC). This study was designed to observe the effect of prenatal alcohol exposure during the second trimester equivalent on the development of dendritic arbors of CC projection neurons (CCpn) in rat visual cortex. In addition, the effect of second trimester equivalent prenatal alcohol exposure on brain weight was determined. Pregnant dams received 1.2-6.0 g/kg ethanol (EtOH) during gestational day (G) 11-20. Controls consisted of normal and nutritionally matched pairfed (PF) dams. Pups were sacrificed on the day of birth, G26, G29 and G33. DiI crystals were placed in the midsagittal CC bundle to retrogradely label CCpn. Images of visual cortex were obtained from tissue slices using a confocal laser scanning microscope. The number and length of apical and basilar dendrite branches were determined. The results show that prenatal alcohol exposure restricted to the second trimester equivalent alters the development of the CCpn dendritic arbor and the brain weight in a blood alcohol concentration (BAC)-dependent manner. The alteration in the EtOH CCpn is manifested as an increase in the number and length of CCpn apical and basilar dendrite branches, while brain weight is reduced compared with Controls.