Actigraphic evidence of persistent sleep disruption following repetitive mild traumatic brain injury in a gyrencephalic model

We studied the effect of multimodal traumatic brain injuries on daily sleep/activity patterns and related histology. Gyrencephalic ferrets wore actigraphs and received military-relevant brain injuries including shockwaves, strong rotational impact, and variable stress, which were evaluated up to 6 months post injury. Sham and Baseline animals exhibited activity patterns occurring in distinct clusters of high activity, interspersed with periods of low activity. In the Injury and Injury + Stress groups, activity clusters diminished and overall activity patterns became significantly more dispersed at 4 weeks post injury with significant sleep fragmentation. Additionally, the Injury + Stress group exhibited a significant decrease in daytime high activity up to 4 months post injury. At 4 weeks post injury, the reactive astrocyte (GFAP) immunoreactivity was significantly greater in both the injury groups compared to Sham, but did not differ at 6 months post injury. The intensity of immunoreactivity of the astrocytic endfeet that surround blood vessels (visualized with aquaporin 4; AQP4), however, differed significantly from Sham at 4 weeks post injury (in both injured groups) and at 6 months (Injury + Stress only). As the distribution of AQP4 plays a key role in the glymphatic system, we suggest that glymphatic disruption occurs in ferrets after the injuries described here.

Evaluation of baseline behavioral tests in ferrets

As the smallest mammal with a gyrencephalic cerebral cortex, ferrets are becoming increasingly important animal models to study neurological disorders. In order for them to be optimally used, typical behavioral measurements are highly desirable. To ascertain a baseline level of behavior, we conducted a battery of tests assessing motor, social, memory, headache, and aspects of depressive-like behavior. Adult male ferrets participated in open field, beam walk, sucrose preference, eye contact, light/dark box, socialization, and novel object recognition tests. The animals were assessed in three cohorts, which differed in age, with the youngest group being approximately 1 year younger than the oldest. Small, but significant, differences occurred between the youngest cohort and the older groups in several areas, suggesting that age may be an important factor when evaluating ferret behavior. Ferrets showed a high level of sociability in the eye contact tests and with novel animal preference. These experiments represent an important baseline of expected normative results that can provide a reference for normal ferret behavior and expected variability. The data reported here may serve as a reference for future intervention studies using the ferret. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Non-Communicable Neurological Disorders and Neuroinflammation

Traumatic brain injury, stroke, and neurodegenerative diseases represent a major cause of morbidity and mortality in Africa, as in the rest of the world. Traumatic brain and spinal cord injuries specifically represent a leading cause of disability in the younger population. Stroke and neurodegenerative disorders predominantly target the elderly and are a major concern in Africa, since their rate of increase among the ageing is the fastest in the world. Neuroimmunology is usually not associated with non-communicable neurological disorders, as the role of neuroinflammation is not often considered when evaluating their cause and pathogenesis. However, substantial evidence indicates that neuroinflammation is extremely relevant in determining the consequences of non-communicable neurological disorders, both for its protective abilities as well as for its destructive capacity. We review here current knowledge on the contribution of neuroinflammation and neuroimmunology to the pathogenesis of traumatic injuries, stroke and neurodegenerative diseases, with a particular focus on problems that are already a major issue in Africa, like traumatic brain injury, and on emerging disorders such as dementias.

Translationally Relevant Magnetic Resonance Imaging Markers in a Ferret Model of Closed Head Injury

Pre-clinical models of traumatic brain injury (TBI) have been the primary experimental tool for understanding the potential mechanisms and cellular alterations that follow brain injury, but the human relevance and translational value of these models are often called into question. Efforts to better recapitulate injury biomechanics and the use of non-rodent species with neuroanatomical similarities to humans may address these concerns and promise to advance experimental studies toward clinical impact. In addition to improving translational aspects of animal models, it is also advantageous to establish pre-clinical outcomes that can be directly compared with the same outcomes in humans. Non-invasive imaging and particularly MRI is promising for this purpose given that MRI is a primary tool for clinical diagnosis and at the same time increasingly available at the pre-clinical level. The objective of this study was to identify which commonly used radiologic markers of TBI outcomes can be found also in a translationally relevant pre-clinical model of TBI. The ferret was selected as a human relevant species for this study with folded cortical geometry and relatively high white matter content and the closed head injury model of engineered rotation and acceleration (CHIMERA) TBI model was selected for biomechanical similarities to human injury. A comprehensive battery of MRI protocols based on common data elements (CDEs) for human TBI was collected longitudinally for the identification of MRI markers and voxelwise analysis of T2, contrast enhancement and diffusion tensor MRI values. The most prominent MRI findings were consistent with focal hemorrhage and edema in the brain stem region following high severity injury as well as vascular and meningeal injury evident by contrast enhancement. While conventional MRI outcomes were not highly conspicuous in less severe cases, quantitative voxelwise analysis indicated diffusivity and anisotropy alterations in the acute and chronic periods after TBI. The main conclusions of this study support the translational relevance of closed head TBI models in intermediate species and identify brain stem and meningeal vulnerability. Additionally, the MRI findings highlight a subset of CDEs with promise to bridge pre-clinical studies with human TBI outcomes.

Expression of GFAP and Tau Following Blast Exposure in the Cerebral Cortex of Ferrets

Blast exposures are a hallmark of contemporary military conflicts. We need improved preclinical models of blast traumatic brain injury for translation of pharmaceutical and therapeutic protocols. Compared with rodents, the ferret brain is larger, has substantial sulci, gyri, a higher white to gray matter ratio, and the hippocampus in a ventral position; these attributes facilitate comparison with the human brain. In this study, ferrets received compressed air shock waves and subsequent evaluation of glia and forms of tau following survival of up to 12 weeks. Immunohistochemistry and Western blot demonstrated altered distributions of astrogliosis and tau expression after blast exposure. Many aspects of the astrogliosis corresponded to human pathology: increased subpial reactivity, gliosis at gray-white matter interfaces, and extensive outlining of blood vessels. MRI analysis showed numerous hypointensities occurring in the 12-week survival animals, appearing to correspond to luminal expansions of blood vessels. Changes in forms of tau, including phosphorylated tau, and the isoforms 3R and 4R were noted using immunohistochemistry and Western blot in specific regions of the cerebral cortex. Of particular interest were the 3R and 4R isoforms, which modified their ratio after blast. Our data strongly support the ferret as an animal model with highly translational features to study blast injury.

KCC2 Manipulation Alters Features of Migrating Interneurons in Ferret Neocortex

KCC2 is a brain specific chloride-potassium cotransporter affecting neuronal development including migration and cellular maturation. It modulates chloride homeostasis influencing the switch of GABA from depolarizing to hyperpolarizing, which contributes to the cues that influence the termination of neuronal migration. The expression of KCC2 during migration of interneurons, therefore, correlates with the ability of these cells to respond to GABA as a stop signal. Manipulation of KCC2 in development can affect various aspects of migrating neurons, including the speed. We describe the effect of KCC2 downregulation and inhibition on features of migrating interneurons of normal ferret kits and those treated with methylazoxymethanol acetate, which increases KCC2. Treatment of organotypic cultures with Bisphenol A, an environmental toxin that alters gene expression, also downregulates KCC2 protein. In organotypic slices treated with the KCC2 antagonist VU0240551, chloride imaging shows inhibition of KCC2 via blockade of chloride flux. Time-lapse video imaging of organotypic cultures treated with either drug, shows a significant increase in the average speed, step size, and number of turns made by migrating neurons leaving the ganglionic eminence. Our findings demonstrate the harmful effect of environmental toxins on brain development and potential consequences in the pathogenesis of neurodevelopmental disorders.

The effect of Zika virus infection in the ferret

Although initial observations of infections with the Zika virus describe a mild illness, more recent reports show that infections by Zika result in neurotropism. In 2015, substantial congenital malformations were observed, with numerous infants born with microcephaly in Brazil. To study the underlying mechanism and effects of the disease, it is critical to find suitable animal models. Rodents lack an immune system parallel to humans and also have lissencephalic brains, which are likely to react differently to infections. As the smallest gyrencephalic mammal, ferrets may provide an important animal model to study the Zika virus, as their brains share many characteristics with humans. To evaluate the prospect of using ferrets to study Zika virus infection, we injected seven pregnant jills with the PR strain subcutaneously on gestational day 21, corresponding to the initiation of corticogenesis. These injections resulted in mixed effects. Two animals died of apparent infection, and all kits were resorbed in another animal that did not die. The other four animals remained pregnant until gestational day 40, when the kits were delivered by caesarian section. We evaluated the animals using CT, MRI, diffusion tensor imaging, and immunohistochemistry. The kits displayed a number of features compatible with an infection that impacted both the brain and skull. The outcomes, however, were variable and differed within and across litters, which ranged from the absence of observable abnormalities to prominent changes, suggesting differential vulnerability of kits to infection by the Zika virus or to subsequent mechanisms of neurodevelopmental disruption.

Viral infections of the central nervous system in Africa

Viral infections are a major cause of human central nervous system infection, and may be associated with significant mortality, and long-term sequelae. In Africa, the lack of effective therapies, limited diagnostic and human resource facilities are especially in dire need. Most viruses that affect the central nervous system are opportunistic or accidental pathogens. Some of these viruses were initially considered harmless, however they have now evolved to penetrate the nervous system efficiently and exploit neuronal cell biology thus resulting in severe illness. A number of potentially lethal neurotropic viruses have been discovered in Africa and over the course of time shown their ability to spread wider afield involving other continents leaving a devastating impact in their trail. In this review we discuss key viruses involved in central nervous system disease and of major public health concern with respect to Africa. These arise from the families of Flaviviridae, Filoviridae, Retroviridae, Bunyaviridae, Rhabdoviridae and Herpesviridae. In terms of the number of cases affected by these viruses, HIV (Retroviridae) tops the list for morbidity, mortality and long term disability, while the Rift Valley Fever virus (Bunyaviridae) is at the bottom of the list. The most deadly are the Ebola and Marburg viruses (Filoviridae). This review describes their epidemiology and key neurological manifestations as regards the central nervous system such as meningoencephalitis and Guillain-Barré syndrome. The potential pathogenic mechanisms adopted by these viruses are debated and research perspectives suggested.

Detection and Distinction of Mild Brain Injury Effects in a Ferret Model Using Diffusion Tensor MRI (DTI) and DTI-Driven Tensor-Based Morphometry (D-TBM)

Mild traumatic brain injury (mTBI) is highly prevalent but lacks both research tools with adequate sensitivity to detect cellular alterations that accompany mild injury and pre-clinical models that are able to robustly mimic hallmark features of human TBI. To address these related challenges, high-resolution diffusion tensor MRI (DTI) analysis was performed in a model of mild TBI in the ferret - a species that, unlike rodents, share with humans a gyrencephalic cortex and high white matter (WM) volume. A set of DTI image analysis tools were optimized and implemented to explore key features of DTI alterations in ex vivo adult male ferret brains (n = 26), evaluated 1 day to 16 weeks after mild controlled cortical impact (CCI). Using template-based ROI analysis, lesion overlay mapping and DTI-driven tensor-based morphometry (D-TBM) significant differences in DTI and morphometric values were found and their dependence on time after injury evaluated. These observations were also qualitatively compared with immunohistochemistry staining of neurons, astrocytes, and microglia in the same tissue. Focal DTI abnormalities including reduced cortical diffusivity were apparent in 12/13 injured brains with greatest lesion extent found acutely following CCI by ROI overlay maps and reduced WM FA in the chronic period was observed near to the CCI site (ANOVA for FA in focal WM: time after CCI p = 0.046, brain hemisphere p = 0.0012) often in regions without other prominent MRI abnormalities. Global abnormalities were also detected, especially for WM regions, which demonstrated reduced diffusivity (ANOVA for Trace: time after CCI p = 0.007) and atrophy that appeared to become more extensive and bilateral with longer time after injury (ANOVA for D-TBM Log of the Jacobian values: time after CCI p = 0.007). The findings of this study extend earlier work in rodent models especially by evaluation of focal WM abnormalities that are not influenced by partial volume effects in the ferret. There is also substantial overlap between DTI and morphometric findings in this model and those from human studies of mTBI implying that the combination of DTI tools with a human-similar model system can provide an advantageous and informative approach for mTBI research.

Emerging Viral Infections in Sub-Saharan Africa and the Developing Nervous System: A Mini Review

The global public health concern is heightened over the increasing number of emerging viruses, i.e., newly discovered or previously known that have expanded into new geographical zones. These viruses challenge the health-care systems in sub-Saharan Africa (SSA) countries from which several of them have originated and been transmitted by insects worldwide. Some of these viruses are neuroinvasive, but have been relatively neglected by neuroscientists. They may provide experiments by nature to give a time window for exposure to a new virus within sizeable, previously non-infected human populations, which, for instance, enables studies on potential long-term or late-onset effects on the developing nervous system. Here, we briefly summarize studies on the developing brain by West Nile, Zika, and Chikungunya viruses, which are mosquito-borne and have spread worldwide out of SSA. They can all be neuroinvasive, but their effects vary from malformations caused by prenatal infections to cognitive disturbances following perinatal or later infections. We also highlight Ebola virus, which can leave surviving children with psychiatric disturbances and cause persistent infections in the non-human primate brain. Greater awareness within the neuroscience community is needed to emphasize the menace evoked by these emerging viruses to the developing brain. In particular, frontline neuroscience research should include neuropediatric follow-up studies in the field on long-term or late-onset cognitive and behavior disturbances or neuropsychiatric disorders. Studies on pathogenetic mechanisms for viral-induced perturbations of brain maturation should be extended to the vulnerable periods when neurocircuit formations are at peaks during infancy and early childhood.

Distribution and Cellular Localization of KCC2 in the Ferret Neocortex

KCC2 (a brain-specific potassium-chloride cotransporter) affects development of the cerebral cortex, including aspects of neuronal migration and cellular maturation and differentiation. KCC2 also modulates chloride homeostasis by influencing the switch of GABA from depolarizing in young neurons to hyperpolarizing in mature neurons. We describe the expression pattern, regional distribution, and cellular colocalization of KCC2 in the ferret cortex in normal kits and those treated with methylazoxymethanol acetate (MAM). We earlier developed a model of impaired cortical development by injecting MAM during mid-cortical gestation, which briefly interferes with neuronal production and additionally results in increased levels of KCC2 at P0. Using immunohistochemistry, we show a shift in KCC2 expression during development, being high in the subplate at P0, repositioning into a subtle laminar pattern in the neocortex at P7-P14, and becoming homogeneous at P35. KCC2 colocalizes with neuronal markers in the developing and mature cerebral cortex of normal ferrets and those treated with MAM, but shows a differential pattern of expression at different ages and locates in distinct cellular compartments during development. Subcellular localization shows that KCC2 predominantly situates in the membrane fraction of neocortical samples. These findings reveal that KCC2 colocalizes differentially with neurons and its expression pattern alters during development.

Progression of histopathological and behavioral abnormalities following mild traumatic brain injury in the male ferret

White matter damage is an important consequence of traumatic brain injury (TBI) in humans. Unlike rodents, ferrets have a substantial amount of white matter and a gyrencephalic brain; therefore, they may represent an ideal small mammal model to study human-pertinent consequences of TBI. Here we report immunohistochemical and behavioral results after a controlled cortical impact (CCI) injury to the sensorimotor cortex of adult male ferrets. We assessed inflammation in the neocortex and white matter, and behavior at 1 day post injury and 1, 4, and 16 weeks post injury (WPI). CCI in the ferret produced inflammation that originated in the neocortex near the site of the injury and progressed deep into the white matter with time. The density of microglia and astrocytes increased in the neocortex near the injury, peaking at 4WPI and remaining elevated at 16WPI. Microglial morphology in the neocortex was significantly altered in the first 4 weeks, but showed a return toward normal at 16 weeks. Clusters of microglial cells in the white matter persisted until 16WPI. We assessed motor and cognitive behavior using the open field, novel object recognition, T-maze, and gait tests. A transient deficit in memory occurred at 4WPI, with a reduction of rearing and motor ability at 12 and 16WPI. Behavioral impairments coincide with features of the inflammatory changes in the neocortex revealed by immunohistochemistry. The ferret represents an important animal model to explore ongoing damage in the white matter and cerebral cortex after TBI.

Diffusion MRI and the detection of alterations following traumatic brain injury

This article provides a review of brain tissue alterations that may be detectable using diffusion magnetic resonance imaging MRI (dMRI) approaches and an overview and perspective on the modern dMRI toolkits for characterizing alterations that follow traumatic brain injury (TBI). Noninvasive imaging is a cornerstone of clinical treatment of TBI and has become increasingly used for preclinical and basic research studies. In particular, quantitative MRI methods have the potential to distinguish and evaluate the complex collection of neurobiological responses to TBI arising from pathology, neuroprotection, and recovery. dMRI provides unique information about the physical environment in tissue and can be used to probe physiological, architectural, and microstructural features. Although well‐established approaches such as diffusion tensor imaging are known to be highly sensitive to changes in the tissue environment, more advanced dMRI techniques have been developed that may offer increased specificity or new information for describing abnormalities. These tools are promising, but incompletely understood in the context of TBI. Furthermore, model dependencies and relative limitations may impact the implementation of these approaches and the interpretation of abnormalities in their metrics. The objective of this paper is to present a basic review and comparison across dMRI methods as they pertain to the detection of the most commonly observed tissue and cellular alterations following TBI.

Population based MRI and DTI templates of the adult ferret brain and tools for voxelwise analysis

Non-invasive imaging has the potential to play a crucial role in the characterization and translation of experimental animal models to investigate human brain development and disorders, especially when employed to study animal models that more accurately represent features of human neuroanatomy. The purpose of this study was to build and make available MRI and DTI templates and analysis tools for the ferret brain as the ferret is a well-suited species for pre-clinical MRI studies with folded cortical surface, relatively high white matter volume and body dimensions that allow imaging with pre-clinical MRI scanners. Four ferret brain templates were built in this study – in-vivo MRI and DTI and ex-vivo MRI and DTI – using brain images across many ferrets and region of interest (ROI) masks corresponding to established ferret neuroanatomy were generated by semi-automatic and manual segmentation. The templates and ROI masks were used to create a web-based ferret brain viewing software for browsing the MRI and DTI volumes with annotations based on the ROI masks. A second objective of this study was to provide a careful description of the imaging methods used for acquisition, processing, registration and template building and to demonstrate several voxelwise analysis methods including Jacobian analysis of morphometry differences between the female and male brain and bias-free identification of DTI abnormalities in an injured ferret brain. The templates, tools and methodological optimization presented in this study are intended to advance non-invasive imaging approaches for human-similar animal species that will enable the use of pre-clinical MRI studies for understanding and treating brain disorders.

Analysis of the effects of noise, DWI sampling, and value of assumed parameters in diffusion MRI models

Purpose

This study was a systematic evaluation across different and prominent diffusion MRI models to better understand the ways in which scalar metrics are influenced by experimental factors, including experimental design (diffusion‐weighted imaging [DWI] sampling) and noise.

Methods

Four diffusion MRI models—diffusion tensor imaging (DTI), diffusion kurtosis imaging (DKI), mean apparent propagator MRI (MAP‐MRI), and neurite orientation dispersion and density imaging (NODDI)—were evaluated by comparing maps and histogram values of the scalar metrics generated using DWI datasets obtained in fixed mouse brain with different noise levels and DWI sampling complexity. Additionally, models were fit with different input parameters or constraints to examine the consequences of model fitting procedures.

Results

Experimental factors affected all models and metrics to varying degrees. Model complexity influenced sensitivity to DWI sampling and noise, especially for metrics reporting non‐Gaussian information. DKI metrics were highly susceptible to noise and experimental design. The influence of fixed parameter selection for the NODDI model was found to be considerable, as was the impact of initial tensor fitting in the MAP‐MRI model.

Conclusion

Across DTI, DKI, MAP‐MRI, and NODDI, a wide range of dependence on experimental factors was observed that elucidate principles and practical implications for advanced diffusion MRI. Magn Reson Med 78:1767–1780, 2017. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Prenatal carbon monoxide impairs migration of interneurons into the cerebral cortex

Prenatal exposure to carbon monoxide (CO) disrupts brain development, however little is known about effects on neocortical maturation. We exposed pregnant mice to CO from embryonic day 7 (E7) until birth. To study the effect of CO on neuronal migration into the neocortex we injected BrdU during corticogenesis and observed misplaced BrdU+ cells. The majority of cells not in their proper layer colocalized with GAD65/67, suggesting impairment of interneuron migration; interneuron subtypes were also affected. We subsequently followed interneuron migration from E15 organotypic cultures of mouse neocortex exposed to CO; the leading process length of migrating neurons diminished. To examine an underlying mechanism, we assessed the effects of CO on the cellular cascade mediating the cytoskeletal protein vasodilator-stimulated phosphoprotein (VASP). CO exposure resulted in decreased cGMP and in a downstream target, phosphorylated VASP. Organotypic cultures grown in the presence of the phosphodiesterase inhibitor IBMX resulted in a recovery of the leading processes. These data support the idea that CO acts as a signaling molecule and impairs function and neuronal migration by acting through the CO/NO-cGMP pathway. In addition, treated mice demonstrated functional impairment in behavioral tests.

Transplanted Neural Progenitor Cells from Distinct Sources Migrate Differentially in an Organotypic Model of Brain Injury

Brain injury is a major cause of long-term disability. The possibility exists for exogenously derived neural progenitor cells to repair damage resulting from brain injury, although more information is needed to successfully implement this promising therapy. To test the ability of neural progenitor cells (NPCs) obtained from rats to repair damaged neocortex, we transplanted neural progenitor cell suspensions into normal and injured slice cultures of the neocortex acquired from rats on postnatal day 0–3. Donor cells from E16 embryos were obtained from either the neocortex, including the ventricular zone (VZ) for excitatory cells, ganglionic eminence (GE) for inhibitory cells or a mixed population of the two. Cells were injected into the ventricular/subventricular zone (VZ/SVZ) or directly into the wounded region. Transplanted cells migrated throughout the cortical plate with GE and mixed population donor cells predominately targeting the upper cortical layers, while neocortically derived NPCs from the VZ/SVZ migrated less extensively. In the injured neocortex, transplanted cells moved predominantly into the wounded area. NPCs derived from the GE tended to be immunoreactive for GABAergic markers while those derived from the neocortex were more strongly immunoreactive for other neuronal markers such as MAP2, TUJ1, or Milli-Mark. Cells transplanted in vitro acquired the electrophysiological characteristics of neurons, including action potential generation and reception of spontaneous synaptic activity. This suggests that transplanted cells differentiate into neurons capable of functionally integrating with the host tissue. Together, our data suggest that transplantation of neural progenitor cells holds great potential as an emerging therapeutic intervention for restoring function lost to brain damage.

Culturing layer-specific neocortical neurons as a cell replacement therapy following traumatic brain injury

Neurophysiological changes resulting from traumatic brain injury (TBI) can result in adverse changes in behavior including mood instability and cognitive dysfunction. Cell death following TBI likely contributes to these altered behaviors and remains an elusive but attractive target for therapies aiming at functional recovery. Previously we demonstrated that neural progenitor cells derived from embryonic rats can be transplanted into donor neonatal rat brain slices and, over the course of 2 weeks in culture, mature into neurons that express neuronal immunohistochemical markers and develop electrophysiological profiles consistent with excitatory and inhibitory interneurons. Here we examine the potential of generating electrophysiologically mature neurons with a layer-specific phenotype as a next step in developing a therapy designed to rebuild a damaged cortical column with the functionally appropriate neuronal subtypes. Preliminary results suggest that neurons derived from passaged neurospheres and grown in dissociated cell culture develop GABAergic and presumed glutamatergic phenotypes and that the percentage of GABAergic cells increases as a function of passage. After 2 weeks in culture, the neurons have a mix of immature and mature neuronal electrophysiological profiles and receive synaptic inputs from surrounding neurons. Subsets of cells expressing neuron specific markers also express layer-specific markers such as Cux1, ER81, and RORβ. Future studies will investigate the potential of transplanting layer-specific neurons generated and isolated in vitro into the neocortex of neonatal brain slices and their potential to maintain their phenotype and integrate into the host tissue.

Targeted disruption of layer 4 during development increases GABAA receptor neurotransmission in the neocortex

Cortical dysplasia (CD) associates with clinical pathologies, including epilepsy and mental retardation. CD results from impaired migration of immature neurons to their cortical targets, leading to clustering of neural cells and changes in cortical properties. We developed a CD model by administering methylazoxymethanol (MAM), an anti-mitotic, to pregnant ferrets on embryonic day 33; this leads to reduction in cortical thickness in addition to redistribution and increased expression of GABAA receptors (GABAAR). We evaluated the impact of MAM treatment on GABAAR-mediated synaptic transmission in postnatal day 0-1 neurons, leaving the ganglionic eminence (GE) and in layer 2/3 pyramidal cells of postnatal day 28-38 ferrets. Embryonic day 33 MAM treatment significantly increases the amplitude and frequency of spontaneous GABAAR-mediated inhibitory postsynaptic currents (IPSCs) in the cells leaving the GE. In older MAM-treated animals, the amplitude and frequency of GABAAR-mediated spontaneous IPSCs in layer 2/3 pyramidal cells is increased, as are the amplitude and frequency of miniature IPSCs. The kinetics of GABAAR opening also altered following treatment with MAM. Western blot analysis shows that the expression of the GABAAα3R and GABAAγ2R subunits amplified in our model animals. We did not observe any significant change in the passive properties of either the layer 2/3 pyramidal cells or cells leaving the GE after MAM treatment. These observations reinforce the idea that synaptic neurotransmission through GABAAR enhances following treatment with MAM and coincides with our finding of increased GABAAαR expression within the upper cortical layers. Overall, we demonstrate that small amounts of toxins delivered during corticogenesis can result in long-lasting changes in ambient expression of GABAAR that influence intrinsic neuronal properties.

Fine-tuning of neurogenesis is essential for the evolutionary expansion of the cerebral cortex

We used several animal models to study global and regional cortical surface expansion: The lissencephalic mouse, gyrencephalic normal ferrets, in which the parietal cortex expands more than the temporal cortex, and moderately lissencephalic ferrets, showing a similar degree of temporal and parietal expansion. We found that overall cortical surface expansion is achieved when specific events occur prior to surpragranular layer formation. (1) The subventricular zone (SVZ) shows substantial growth, (2) the inner SVZ contains an increased number of outer radial glia and intermediate progenitor cells expressing Pax6, and (3) the outer SVZ contains a progenitor cell composition similar to the combined VZ and inner SVZ. A greater parietal expansion is also achieved by eliminating the latero-dorsal neurogenic gradient, so that neurogenesis displays a similar developmental degree between parietal and temporal regions. In contrast, mice or lissencephalic ferrets show more advanced neurogenesis in the temporal region. In conclusion, we propose that global and regional cortical surface expansion rely on similar strategies consisting in altering the timing of neurogenic events prior to the surpragranular layer formation, so that more progenitor cells, and ultimately more neurons, are produced. This hypothesis is supported by findings from a ferret model of lissencephaly obtained by transiently blocking neurogenesis during the formation of layer IV.

Migratory behavior of cells generated in ganglionic eminence cultures

Migration of cells is a common process that leads to the development and maturation of the vertebrate central nervous system (Hatten, '99). The cerebral cortex consists of two basic neuronal types: excitatory and inhibitory. These cells arise in distinct areas and migrate into the cortex along different routes (Pearlman et al., '98). Inhibitory interneurons migrate tangentially from subcortical sources, mostly from different regions of the ganglionic eminences (Gelman et al., '09; Xu et al., '04). Their movement requires precise spatiotemporal control imposed by environmental cues, to allow for the establishment of proper cytoarchitecture and connectivity in the cerebral cortex (Caviness & Rakic, '78; Hatten, '90; Rakic, '90). To study the migratory behavior of cells generated in proliferative zones of the ganglionic eminences (GE) in newborn ferrets in vitro we used a 3 dimensional culture arrangement in a BD Matrigel Matrix. The culture setup consisted of two GE explants and a source of tested proteins extracted from the cerebral cortex and adsorbed on fluorescent latex Retrobeads IX positioned between the explants (Hasling et al., '03; Riddle et al., '97). After 2-3 days of culture, the cells start to appear at the edge of the explant showing a propensity to leave the tissue in a radial direction. Live imaging allowed observation of migratory patterns without the necessity of labeling or marking the cells. When exposed to fractions of the protein extract obtained from isochronic ferret cortex, the GE cells displayed different behaviors as judged by quantitative kinetic analysis of individual moving cells.

Populations of radial glial cells respond differently to reelin and neuregulin1 in a ferret model of cortical dysplasia

Radial glial cells play an essential role during corticogenesis through their function as neural precursors and guides of neuronal migration. Both reelin and neuregulin1 (NRG1) maintain the radial glial scaffold; they also induce expression of Brain Lipid Binding Protein (BLBP), a well known marker of radial glia. Although radial glia in normal ferrets express both vimentin and BLBP, this coexpression diverges at P3; vimentin is expressed in the radial glial processes, while BLBP appears in cells detached from the ventricular zone. Our lab developed a model of cortical dysplasia in the ferret, resulting in impaired migration of neurons into the cortical plate and disordered radial glia. This occurs after exposure to the antimitotic methylazoxymethanol (MAM) on the 24th day of development (E24). Ferrets treated with MAM on E24 result in an overall decrease of BLBP expression; radial glia that continue to express BLBP, however, show only mild disruption compared with the strongly disrupted vimentin expressing radial glia. When E24 MAM-treated organotypic slices are exposed to reelin or NRG1, the severely disrupted vimentin+ radial glial processes are repaired but the slightly disordered BLBP+ processes are not. The realignment of vimentin+ processes was linked with an increase of their BLBP expression. BLBP expressing radial glia are distinguished by being both less affected by MAM treatment and by attempts at repair. We further investigated the effects induced by reelin and found that signaling was mediated via VLDLR/Dab1/Pi3K activation while NRG1 signaling was mediated via erbB3/erbB4/Pi3K. We then tested whether radial glial repair correlated with improved neuronal migration. Repairing the radial glial scaffold is not sufficient to restore neuronal migration; although reelin improves migration of neurons toward the cortical plate signaling through ApoER2/Dab1/PI3K activation, NRG1 does not.

On the biomarkers and mechanisms of konzo, a distinct upper motor neuron disease associated with food (cassava) cyanogenic exposure

Konzo is a self-limiting central motor-system disease associated with food dependency on cassava and low dietary intake of sulfur amino acids (SAA). Under conditions of SAA-deficiency, ingested cassava cyanogens yield metabolites that include thiocyanate and cyanate, a protein-carbamoylating agent. We studied the physical and biochemical modifications of rat serum and spinal cord proteins arising from intoxication of young adult rats with 50-200mg/kg linamarin, or 200mg/kg sodium cyanate (NaOCN), or vehicle (saline) and fed either a normal amino acid- or SAA-deficient diet for up to 2 weeks. Animals under SAA-deficient diet and treatment with linamarin or NaOCN developed hind limb tremors or motor weakness, respectively. LC/MS-MS analysis revealed differential albumin carbamoylation in animals treated with NaOCN, vs. linamarin/SAA-deficient diet, or vehicle. 2D-DIGE and MALDI-TOF/MS-MS analysis of the spinal cord proteome showed differential expression of proteins involved in oxidative mechanisms (e.g. peroxiredoxin 6), endocytic vesicular trafficking (e.g. dynamin 1), protein folding (e.g. protein disulfide isomerase), and maintenance of the cytoskeleton integrity (e.g. α-spectrin). Studies are needed to elucidate the role of the aformentioned modifications in the pathogenesis of cassava-associated motor-system disease.

Regulation of neural progenitor cell development in the nervous system

The mammalian telencephalon, which comprises the cerebral cortex, olfactory bulb, hippocampus, basal ganglia, and amygdala, is the most complex and intricate region of the CNS. It is the seat of all higher brain functions including the storage and retrieval of memories, the integration and processing of sensory and motor information, and the regulation of emotion and drive states. In higher mammals such as humans, the telencephalon also governs our creative impulses, ability to make rational decisions, and plan for the future. Despite its massive complexity, exciting work from a number of groups has begun to unravel the developmental mechanisms for the generation of the diverse neural cell types that form the circuitry of the mature telencephalon. Here, we review our current understanding of four aspects of neural development. We first begin by providing a general overview of the broad developmental mechanisms underlying the generation of neuronal and glial cell diversity in the telencephalon during embryonic development. We then focus on development of the cerebral cortex, the most complex and evolved region of the brain. We review the current state of understanding of progenitor cell diversity within the cortical ventricular zone and then describe how lateral signaling via the Notch-Delta pathway generates specific aspects of neural cell diversity in cortical progenitor pools. Finally, we review the signaling mechanisms required for development, and response to injury, of a specialized group of cortical stem cells, the radial glia, which act both as precursors and as migratory scaffolds for newly generated neurons.

Migration of transplanted neural progenitor cells in a ferret model of cortical dysplasia

Although altered gene expression clearly causes failure of the neocortex to form properly, many causes of neocortical dysplasia arise from environmental or unknown factors. Our lab studies a model of cortical dysplasia induced by injection of methylazoxymethanol (MAM) into pregnant ferrets on embryonic day 33 (E33), which shares many features of neocortical dysplasia in humans. E33 MAM treatment results in characteristic deficits that include dramatic reduction of layer 4 in somatosensory cortex, widespread termination of thalamic afferents, and altered distribution of GABAergic elements. We determined the ability of immature cells to migrate into MAM-treated cortex using ferret neural progenitor cells obtained at E27 and E33 and mouse neural progenitor cells obtained at E14. When these cells were transplanted into organotypic cultures obtained from normal and E33 MAM-treated ferret cortex prepared on postnatal day 0 (P0), all progenitor cells migrated similarly in both hosts, preferentially residing in the upper cortical plate. The site of transplantation was significant, however, so that injections into the ventricular zone were more likely to reach the cortical plate than transplants into the intermediate zone. When similar cells were transplanted into ferret kits, approximately P7-P9, and allowed to survive for 2-4 weeks, the donor cells migrated differently and also reached distinct destinations in normal and MAM-treated hosts. MAM-treated cortex was more permissive to invasion by donor cells as they migrated to widespread aspects of the cortex, whereas transplants in normal host cortex were more restricted. E27 neural progenitor cells populated more cortical layers than later born E33 neural progenitor cells, suggesting that the fate of transplanted cells is governed by a combination of extrinsic and intrinsic factors.

A normal radial glial scaffold is necessary for migration of interneurons during neocortical development

The relationship between radial glia and neurons migrating tangentially from the ganglionic eminence (GE) has been suggested but not firmly established. To study this relationship we used a ferret model of cortical dysplasia where radial glia are highly disorganized. To produce this, an antimitotic, methylazoxy methanol (MAM) is injected on the 24th day of gestation (E24 MAM). Neurons migrating away from the GE in MAM-treated animals tend to remain in the intermediate zone (IZ) and do not reach the cortical plate (CP) as they do in normal ferret slices. We recently observed that the disrupted radial glia after MAM treatment could be restored toward their normal morphology by exogenous application of neuregulin1 (NRG1). We demonstrate here that when E24 MAM slices are treated with NRG1, the distribution of cells arising from the GE was similar to normal slices. In a second paradigm, we disrupted radial glia by adding ciliary neurotrophic factor (CNTF) to the culture media of normal ferret slices; CNTF induces acute differentiation of radial glia into astrocytes. After CNTF exposure, few tangentially migrating cells reach the CP compared to untreated slices. These results show that interneurons fail to reach the CP by disrupted normal radial glia and restoring the normal radial glial scaffold is sufficient to allow migrating cells to invade the CP. Our results suggest an important role for radial glia by controlling directly or indirectly the migration of interneurons to the CP, their main target.

Alteration of interneuron migration in a ferret model of cortical dysplasia

During cerebral cortical development, gamma-aminobutyric acidergic (GABAergic) interneurons arise from a different site than projection neurons. GABAergic cells are generated in the subpallial ganglionic eminence (GE), while excitatory projection neurons arise from the neocortical ventricular zone. Our laboratory studies a model of cortical dysplasia that displays specific disruption of GABAergic mechanisms and an alteration in the overall balance of excitation in the neocortex. To produce this model, the birth of neurons on a specific gestational day in ferrets (embryonic day 33 [E33]) is interrupted by injection of the antimitotic methylazoxymethanol (MAM). We hypothesized that migration of interneurons might be disrupted in this cortical dysplasia paradigm. We observed that although interneurons migrate into the neocortex in both normal and dysplastic cortex, the migrating cells become disoriented over time after E33 MAM treatment. Coculture experiments using normal GE and MAM-treated cortex (and vice versa) demonstrate that cues dictating proper orientation of migrating interneurons arise from the cortex and are not intrinsic to the migrating cells. As a consequence, interneurons in mature brains of MAM-treated animals are abnormally distributed. We report that GABA(A) receptor activation is crucial to the proper positioning of interneurons migrating into the cortex from the GE in normal and MAM-treated animals.

Endogenous neuregulin restores radial glia in a (ferret) model of cortical dysplasia

Radial glia are integral components of the developing neocortex. During corticogenesis, they form an important scaffold for neurons migrating into the cortical plate. Recent attention has focused on neuregulin (NRG1), acting through erbB receptors, in maintaining their morphology. We developed a model of developmental radial glial disruption by delivering an antimitotic [methylazoxy methanol (MAM)] to pregnant ferrets on embryonic day 24 (E24). We previously found that normal ferret cortex contains a soluble factor capable of realigning the disorganized radial glia back toward their normal morphology. Characterization of the reorganizing activity in normal cortex demonstrated that the probable factor mediating these responses was a 30-50 kDa protein. To test whether this endogenous soluble factor was NRG1, we used organotypic cultures of E24 MAM-treated ferret neocortex supplemented with the endogenous factor obtained from normal cortical implants, exogenous NRG1beta, antibodies that either blocked or stimulated erbB receptors, or a soluble erbB subtype that binds to available NRG1. We report that exogenous NRG1 or antibodies that stimulate erbB receptors dramatically improve the morphology of disrupted radial glia, whereas blockade of NRG1-erbB signaling prevents the radial glial repair. Our results suggest that NRG1 is an endogenous factor in ferret neocortex capable of repairing damaged radial glia and that it acts via one or more erbB receptors.

Disruption of layer 4 development alters laminar processing in ferret somatosensory cortex

Treatment with the anti-mitotic agent methylazoxymethanol (MAM) on embryonic day 33 (E33) in ferrets changes features of somatosensory cortex. These include dramatic reduction of cells in layer 4, and altered distributions of thalamocortical afferent terminations and GABA(A) receptors. To determine the effect of the relative absence of layer 4 on processing of sensory stimuli we used current source-density profiles to assess laminar activity patterns. Nearly synchronous activation occurs across all layers in treated animals, which contrasts with the normal cortical activation pattern of initial sinks in layer 4. This change after MAM treatment is consistent with the absence of layer 4 cells and widespread termination of thalamocortical afferents. Using periodic stimulation at 'flutter' frequency, layer 4 neurons in normal somatosensory cortex fire reproducibly to the stimulus rate; the capacity for entrainment is best for layer 4 and weaker in the extragranular layers. The capacity to encode periodic sensory stimuli is disrupted in MAM-treated somatosensory cortex; after an initial response to the onset of periodic stimuli, neurons in all cortical layers show weak entrainment. Neural responses to sensory drive in E33 MAM-treated cortex are also embedded in levels of neural activity substantially above those in normal somatosensory cortex. Sustained stimulation additionally reveals different capacities in each layer for improved signal-to-noise ratios, with layer 4 neurons in normal animals exhibiting the most improved signaling over time. We conclude that normal thalamic terminations, an intact layer 4 and subsequent intracortical processing are integral to proper encoding of stimulus features.

GABAA Receptors Reorganize when Layer 4 in Ferret Somatosensory Cortex is Disrupted by Methylazoxymethanol (MAM)

We established a model of cortical development that arrests the birth of layer 4 cells by injecting methylazoxymethanol (MAM) on embryonic day 33 (E33) in ferrets. This leads to adult somatosensory cortex with a very thin layer 4. Earlier, we determined the relative absence of layer 4 changed the growth and differentiation of the somatosensory cortex and the growth of thalamic afferents into the cortical plate. To identify other features of cortical organization that might be altered after MAM treatment, we assessed the distribution of selected excitatory and inhibitory receptors in area 3b of ferret somatosensory cortex. Initial screening revealed the distribution of several excitatory receptors (NMDA, AMPA, kainate) in E33 MAM-treated cortex was similar to that in normal adult animals. In contrast, the binding pattern of inhibitory GABAA receptors was altered in MAM-treated cortex. Normally, GABAA receptors densely locate in central layers of cortex. In E33 MAM-treated animals, GABAA receptor binding extended superficially, covering a broader area of cortex. Further experiments using antibodies directed against GABAAalpha receptors disclosed that pan alpha GABAA receptors strongly localize to layer 4 in normal area 3b. In E33 MAM-treated cortex, however, GABAAalpha receptors extend outside and are located above and below the very thin layer 4. The redistribution of inhibitory receptors suggests that layer 4 plays an important role in regulating thalamic terminations and also in the resulting ability to refine processing of incoming stimuli.

Factors affecting the morphology of radial glia

A model of cortical dysplasia results from disruption of the earliest generated neocortical cells. Injections of an antimitotic (methylazoxy methanol - MAM) into pregnant ferrets result in a constellation of effects, which include disruption of radial glia, with early differentiation in astrocytes, and impaired migration of neurons into the cortical plate. We found previously that culture of P0 MAM-treated slices with explants of normal cortical plate reorganizes the radial glia toward their normal morphology and improves migration of neurons into the cortical plate. This suggested that P0 normal cortical plate contains a 'factor' capable of providing reorganizing cues to disorganized developing cortex. The current study characterizes the biological activity in normal cortical plate by isolating fractions of different molecular weight obtained from conditioned media of organotypic cultures. The only media fraction capable of providing reorganizing activity to MAM-treated cortex was the molecular weight fraction between 30 and 50 kDa. Treatment designed to denature proteins demonstrated that the active molecular weight fraction (30-50 kDa) was not able to provide reorganizing cues when either heated or treated with Proteinase K. These data provide support for the idea that normal cortical plate of neonatal ferret contains a radialization factor that is a protein of 30-50 kDa.

Influence of radial glia and Cajal-Retzius cells in neuronal migration

Normal development of cerebral cortex depends on proper sequential genesis of cortical neurons and glia. Disruption of corticogenesis in ferret by short-term arresting of cell division using injections of methylazoxy methanol (MAM) leads to a specific constellation of effects, including disruption and early differentiation of radial glia into astrocytes and disorganization of reelin-containing Cajal-Retzius cells. We hypothesize that early interference of normal cortical development removes a factor instrumental in maintaining radial glia in their normal elongated shape. In support of this idea, coculture of MAM-treated slices with normal cortical plate restores radial glia and Cajal-Retzius cells to their normal positions. Recently, we found that conditioned medium obtained from normal organotypic cultures returned radial glia toward their normal morphology only in a fraction of 30-50 kDa molecular weight (MW). To assess whether restoring this factor would also improve effective migration into the cortical plate of E24 MAM-treated animals, we conducted experiments using cocultures of normal cortical plate with organotypic cultures of MAM-treated cortex, which received prior BrdU injections. In both the normal and E24 MAM-treated/normal cortical plate coculture, a greater percentage of BrdU positive cells migrated effectively into the cortical plate. We suggest that early interruption of cell division eliminates a population of cells and a factor important for maintaining proper cortical development, specifically providing cues maintaining elongation of radial glia.

Developmental regulation of plasticity in the forepaw representation of ferret somatosensory cortex

This study characterized the spatiotemporal responses in ferret somatosensory cortex after sensory deprivation at different phases of cortical development. We hypothesized that cortical responses to stimulation of intact superficial radial nerve in adults will vary systematically according to maturation of thalamocortical relationships at the time of an ulnar nerve transection. Depending on the age of the animal at the time of the lesion, we found differential effects on the spatial distribution of the short- and long-latency components of the cortical response. In animals lesioned at postnatal days 5-7, when thalamic projections are not yet stabilized and layer 4 is not yet formed, we found that initial (short-latency) cortical responses are widespread and fragmented. Ulnar nerve transections performed at postnatal day 20 or 21, when thalamocortical afferents are more stabilized and layer 4 is clearly identifiable, yield moderate expansions in the distribution of short- and long-latency components of the cortical response. Nerve lesions in adults lead to a wider distribution of long-latency cortical activity. Neonatal lesions broaden the spatial distribution and increase the latency of the initial cortical response; interruption of nerve input in older juveniles alters both the early and later components; and nerve lesions in adult animals expand the distribution of later cortical activity only. These findings demonstrate correlation between developmental phase at the time sensory input is interrupted and the latency of affected components of the cortical response. This supports the hypothesis that differential response changes are regulated by functional reorganization of thalamocortical connections after neonatal lesions and alteration of corticocortical dynamics after adult lesions.

A radialization factor in normal cortical plate restores disorganized radial glia and disrupted migration in a model of cortical dysplasia

Treatment of pregnant ferrets on embryonic day 24 (E24) with the antimitotic methylazoxy methanol (MAM) leads to a specific constellation of effects in newborn kits, which include a very thin and poorly laminated neocortex, disruption of radial glial cell morphology with early differentiation into astrocytes, and abnormal positioning of Cajal-Retzius cells. We suggest that MAM treatment on E24 results in this model of cortical dysplasia by eliminating a population of cells that produce a factor capable of maintaining radial glia in their normal morphology. The abnormal radial glia, either alone or in combination with other abnormal features, are likely to prevent proper migration into the cortical plate. To test the possibility that normal cortex can provide the missing substance that influences radial glia, slices of E24 MAM-treated cortex were removed at postnatal day 0 (P0) and cultured adjacent to explants of P0 normal cortical plate. By labelling a small number of cells with injections of fluorescent dextrans into the cultured slices, we found that abnormal radial glia in MAM treated slices cocultured adjacent to normal cortical plate were restored toward normal, in comparison to E24 MAM treated slices cultured alone and in other control conditions. We also found that abnormally positioned Cajal-Retzius cells move into the marginal zone and that neurons are able to migrate into the cortical plate more effectively in the coculture condition. These data indicate that normal cortical plate of ferrets contains a factor causing radial glia to maintain their elongated morphology; the improved position of radial glia encourages repositioning of Cajal-Retzius cells and improved neuronal migration into the cortical plate.

Transplants of NGF-secreting fibroblasts restore stimulus-evoked activity in barrel cortex of basal-forebrain-lesioned rats

Cholinergic nuclei in the basal forebrain supply the cerebral cortex with acetylcholine (ACh). Depletion of cholinergic fibers following basal forebrain lesion results in reduced stimulus-evoked functional activity in rat barrel cortex in response to whisker stimulation. We showed previously that exogenous delivery of nerve growth factor (NGF) to the lateral ventricle restores reduced functional activity toward normal despite persistent reductions in cortical cholinergic activity. Gene transfer of therapeutic peptides using genetically engineered cells allows for localized and biological delivery of compounds to the CNS, circumventing systemic administration or repetitive invasive surgery. In this study, we grafted genetically engineered fibroblasts that secrete NGF (NGF+) into three CNS loci of rats with unilateral basal forebrain lesions, along with control fibroblasts (NGF-) that did not secrete NGF. Only NGF+ fibroblasts grafted into ACh-depleted somatosensory cortex resulted in improvement of functional activity following cholinergic depletion. NGF+ fibroblast transplants into the lateral ventricle or basal forebrain did not improve functional activity nor did NGF- fibroblasts in any site. Similar to our previous experiments using intraventricular NGF injections, despite improvements in functional activity, the affected barrel cortex remained depleted of acetylcholinesterase-stained fibers following insertion of NGF+ fibroblasts. These data support the idea that NGF can act directly on the cerebral cortex following reductions in cholinergic innervation. The mechanism of NGF action is elusive, however, since the presence of its high-affinity receptor, trkA, in the cerebral cortex is controversial.

Disruption of layers 3 and 4 during development results in altered thalamocortical projections in ferret somatosensory cortex

The precision of projections from dorsal thalamus to neocortex are key toward understanding overall cortical organization and function. To identify the significance of layer 4 cells in receiving the bulk of thalamic projections in somatosensory cortex, we disrupted layer 4 genesis and studied the effect on thalamic terminations in ferrets. Second, we ascertained the result of layer 4 disruption on functional responses and topographic organization. Methylazoxy methanol (MAM) was injected into pregnant ferrets on embryonic day 33 (E33), when most layer 4 neurons of somatosensory cortex are generated. This treatment resulted in dramatic reduction in the thickness of targeted layer 4. E38 MAM treatment was used as a control, when layer 2-3 neurons are generated. The projections of ventrobasal thalamus into somatosensory cortex were studied using DiI injections. We found only subtle differences between groups (normal, E33, or E38 MAM-treated) in the thalamic afferent pattern on postnatal day 1 (P1) and P7. On P14, thalamic terminations distribute almost equally throughout the remaining cortical layers in the E33 MAM-treated group compared with normal and E38 MAM-treated animals, in which the ventrobasal thalamus projects primarily to central layers. Electrophysiological recordings conducted on mature ferrets treated with MAM on E33 demonstrated that somatotopic organization and receptive field size are normal. These findings emphasize the importance of layer 4 in determining the normal laminar pattern of thalamic termination and suggest that, although its absence is likely to impact on complex neocortical functional responses, topographic organization does not arise from the influence of layer 4.

Laminar specific alterations of thalamocortical projections in organotypic cultures following layer 4 disruption in ferret somatosensory cortex

The developing neocortex influences the growth of thalamocortical projections. Layer 4 in particular receives the majority of input from the thalamus and is important in instructing thalamic afferents to terminate. Previous in vivo experiments demonstrated that disruption of layer 4 during corticogenesis in ferret somatosensory cortex by application of methylazoxy methanol acetate (MAM) prevents proper termination of thalamic afferents in appropriate cortical regions. To further explore the role of layer 4 in thalamocortical development, we prepared organotypic cocultures consisting of normal gestational day 0 (P0) ferret thalamus paired with normal, embryonic day 33 (E33), or E38 MAM-treated cortex obtained from ferrets at either P0 or P7. Injection of MAM on E33 disrupts layer 4 formation, whereas similar injections on E38 interfere with layer 2 formation. The cocultures grew together for a number of days, then discrete injections of either fluorescent dextrans or 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) were made into the thalamic piece. The labeled thalamic afferents that grew into the cortical slice were analysed and the sites of their terminations quantified after 3, 5, or 7-10 days in culture (DIC). Our results varied somewhat with the amount of time in culture, but the preponderance of thalamic fibers in normal cortex terminated in layer 4, whereas their counterparts in E33 MAM-treated cortex grew beyond the cortical plate and many fibers terminated inappropriately within lower cortical layers or white matter. Terminal distribution of thalamic fibers in E38 MAM-treated cortex looked similar to normal. These results demonstrate that the cells of layer 4 provide thalamic afferents with important positional and termination cues.

Nerve growth factor increases stimulus-evoked metabolic activity in acetylcholine-depleted barrel cortex

Lesions of the basal forebrain deplete the neocortex of cholinergic fibers. Acetylcholine depletion in the somatosensory cortex of rats results in reduced stimulus-evoked activity in response to whisker stimulation. Previous studies demonstrate that embryonic basal forebrain transplants improve functional activity toward normal. It is not clear if the activity increase is due to cholinergic replacement or other factors present in the graft. In this study, we examined the possibility that nerve growth factor (NGF), a neurotrophin known as a survival factor and a specific protectant for cholinergic basal forebrain neurons, can preserve basal forebrain cells after a lesion and restore functional activity in the somatosensory cortex. We report that NGF alone is capable of restoring functional activity in the barrel cortex of animals with basal forebrain lesions, while vehicle injections of saline do not alter activity. Both high (10 microg) and low (5 microg) doses of NGF unilaterally injected into the lateral ventricle improved stimulus-evoked functional activity during bilateral whisker stimulation. The mechanism of NGF action is not clear since the restoration of functional activity in cortex was not accompanied by increased cholinergic activity as detected by acetylcholinesterase fiber staining. NGF may act directly on cortical neurons, although its site of action is not well defined.

Anatomical correlates of representational map reorganization induced by partial vibrissectomy in the barrel cortex of adult mice

We examined the potential for changes in cortical connectivity to accompany long-term plastic changes in functional cortical representations of mystacial vibrissae. Plasticity in the barrel cortex of young adult mice was evoked by vibrissectomy that spared row C of whiskers. We found that 2-deoxyglucose brain mapping causes a progressive expansion of cortical representation of the spared vibrissae. Two months after vibrissectomy, when the width of the cortical map of the spared row of vibrissae doubled, living cortical slices of the barrel cortex were injected with fluorescent dextrans. The injections were centered on spared, deprived and control vibrissal columns. The injections labeled three intracortical projection systems: (i) local connections from one vibrissal column to neighboring columns; (ii) long-range projections running in the septa and walls of the barrels and spanning several barrels; and (iii) very-long-range fibers running horizontally in the lower part of layer V. The local, short-range projection system was analysed following small injections into the centers of columns in layers III and IV. We found that injections into spared barrels labeled axons extending for significantly greater distances in all layers (except layer V), and labeled cell bodies situated significantly further, than after injections into deprived or control barrels. Also, the total axonal density labeled by injections into the spared barrel was higher by 70% than for the deprived or control barrels. Alterations of topographical maps in adult somatosensory cortex may occur immediately after functional denervation, but may also increase with time, as in the case of our experimental situation. Our results indicate that persistent, long-term plastic change can remodel connectivity in the barrel cortex.

Interference with the development of early generated neocortex results in disruption of radial glia and abnormal formation of neocortical layers

Early generated layers of neocortex are important factors in forming the subsequent architecture of the cerebral cortex. To further explore the role of early generated cortex, we disrupted formation of an early generated cohort of cells by intraperitoneal injections of the mitotic inhibitor methylazoxymethanol (MAM) into pregnant ferrets timed to coincide with generation of subplate neurons in the ventricular zone. Our studies demonstrate that if early development of the neocortex is interrupted by injection of MAM during embryogenesis (on embryonic day 24 or 28; E24 or E28), a distinct laminar pattern fails to form properly in the parietal cortex. A reduced number of MAP2-positive cells were observed in the region of the subplate when compared with the number of MAP2-positive cells found in normal animals. Interference with the superficial neocortical layers that form later during development (on embryonic day 33) by appropriately timed MAM injections does not result in a severely disrupted laminar pattern. The interrupted laminar pattern that arises after early MAM injections coincides with distorted radial glial cells (identified by immunoreactivity to the intermediate filament protein, vimentin), which occur after early, but not late, MAM injections. Further analysis suggests that interference with early development of neocortex leads to premature differentiation of radial glial cells into astrocytes, as demonstrated by the presence of glial fibrillary acidic protein (GFAP). Experiments involving injections of the thymidine analog, bromodeoxyuridine (BRDU), demonstrated that 4 days after E24 MAM injection cells are generated and migrate into the thin cortical plate. By E38, however, cells continue to be generated in animals treated with MAM on E24 but do not reach their normal positions in the cortical plate. In addition, immunoreactivity using the CR50 antibody, which identifies presumptive Cajal-Retzius cells present in layer 1, demonstrates that the CR50-positive cells, normally precisely located in the outer portion of layer 1, are distributed in disarray throughout the thickness of the neocortex and intermediate zone in early MAM-treated animals, but not in those treated with MAM injections later during gestation. These findings are consistent with the idea that early generated layers are important in providing factors that maintain the environment necessary for subsequent neuronal migration and formation of neocortical layers.

Organization of the forepaw representation in ferret somatosensory cortex

Little is known about the function and structure of ferret somatosensory cortex. We used anatomical methods and multi-unit recordings to characterize the cytoarchitecture, functional responses and topography of the forepaw representation in ferret somatosensory cortex. The representation of the cutaneous ferret forepaw encompasses approximately the caudal half of the posterior sigmoid gyrus. The posterior sigmoid gyrus and coronal sulcus contain unique cytoarchitectonic fields that conform in large part to earlier descriptions of somatosensory regions in the cat. The cytoarchitectonic regions form mediolateral bands that comprise areas 4, 3a, 3b, 1, and 2 (from rostral to caudal). We studied most extensively areas 3a and 3b for functional responses to somatic stimuli; our data indicate that ferret somatosensory cortex contains at least two representations of the forepaw in these two areas. Our data also suggest that within ferret somatosensory cortex, the morphological and submodality features gradually, rather than abruptly, distinguish themselves as unique cortical fields.

Histogenesis of ferret somatosensory cortex

The ferret has emerged as an important animal model for the study of neocortical development. Although detailed studies of the birthdates of neurons populating the ferret visual cortex are available, the birthdates of neurons that reside in somatosensory cortex have not been determined. The current study used bromodeoxyuridine to establish when neurons inhabiting the somatosensory cortex are generated in the ferret; some animals also received injections of [3H]thymidine. In contrast to reports of neurogenesis in ferret visual cortex, most neurons populating the somatosensory cortex have been generated by birth. Although components of all somatosensory cortical layers have been produced at postnatal day 0, the layers are not distinctly formed but develop over a period of several weeks. A small number of neurons continue to be produced for a few days postnatally. The majority of cells belonging to a given layer are born over a period of approximately 3 days, although the subplate and last (layer 2) generated layer take somewhat longer. Although neurogenesis of the neocortex begins along a similar time line for visual and somatosensory cortex, the neurons populating the visual cortex lag substantially during the generation of layer 4, which takes more than 1 week for ferret visual cortex. Layer formation in ferret somatosensory cortex follows many established principles of cortical neurogenesis, such as the well-known inside-out development of cortical layers and the rostro-to-caudal progression of cell birth. In comparison with the development of ferret visual cortex, however, the generation of the somatosensory cortex occurs remarkably early and may reflect distinct differences in mechanisms of development between the two sensory areas.

Development of intrinsic connections in cat somatosensory cortex

We explored the development of clustered connections in cat somatosensory cortex by using intracortical injections of biocytin and carbocyanine dye (DiI). Biocytin injections in adults revealed clusters of retrogradely labeled cells, affirming earlier reports of the patchy nature of corticocortical connectivity in the adult cat somatosensory cortex. On postnatal days (PNDs) 1 and 3, a diffuse distribution of axons and labeled cells were found after DiI injections. A dramatic rearrangement of connections had taken place by PND 6. Well-defined clusters of labeled cells surrounded the injection site. Intercluster zones were relatively free of labeled cells and contained few labeled axons. At later ages (PNDs 12 and 15), a clear patchy distribution of intrinsic connections was seen. We analyzed neuronal clustering by using spatial point process analysis, which corroborated our qualitative observations. The density of labeled neurons was significantly higher in clusters than in the intercluster zones on PND 6. Autocorrelations run on profile plots of optical density values, along paths parallel to the edge of the injection sites (reflecting axonal distributions), revealed significant periodicity (P < .05; center-to-center approximately 750 pm) by PND 6. These data demonstrate that corticocortical connections in cat somatosensory cortex develop from a diffuse network at birth, which transforms into a patchy network by the end of the 1st postnatal week. This is substantially earlier than the development of local circuits in the cat visual cortex and may reflect fundamental differences in the organization of the two cortices.

Development of local connections in ferret somatosensory cortex

Ferrets have become recognized as a useful and interesting model for study of neocortical development. Because of their immaturity at birth, it is possible to study very early events in the ontogeny of the brain. We used living slices of ferret somatosensory cortex to study the formation and development of intrinsic elements within the neocortex. A small number of fixed, hemisected brains injected with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) were also used. The slices were obtained from ferret kits aged postnatal day (P)1 to P62 and maintained in a chamber; each slice received injections of fluorescent-labeled dextrans. The injections were made at different ages in several distinct sites, which included the proliferative ventricular zone, the intervening white matter (or intermediate zone), and different sites of developing cortex, including the deeper cortical plate, which incorporated the subplate in young animals, and more superficial cortical sites depending on the age of the animal. Several animals also received injections into the ventrobasal thalamus. Injections into young animals (P1-7) produced a dominant radial pattern that extended from the ventricular zone into the cortex. Injections into the ventricular zone labeled many cells that appeared morphologically like radial glia as well as presumptive neurons. Although the predominant pattern was radial, injections in the ventricular zone often produced tangentially oriented cells and horizontally arranged fibers at the outer edge of the proliferative zone. These cells and fibers may provide a substrate for tangential dispersion of neurons within the neocortex. More superficial injections within the slice labeled lines of cells that appeared to be stacked upon one another in a radial pile in the cortex; the cortical plate received very few lateral projections. Data obtained from more mature slices indicated that although the overall pattern of staining remained radial, the precise character of the pattern changed to include more lateral spread into surrounding cortex, which eventually refined and developed into distinct patches by P28, when the overall cortical architecture appeared adult like. The data involving thalamocortical connections were more limited, but they indicated that the thalamus projects precisely to the somatosensory cortex in a point-to-point fashion from the earliest date studied (P0) and that the ventrobasal nucleus terminates upon the somatosensory cortex in a patchy manner during the early postnatal days of development. This study of the development of the somatosensory cortex confirms the ubiquitous nature of column-like connections throughout the neocortex and provides a novel view of the radial nature of early neocortical maturation.

The impact of basal forebrain lesions on the ability of rats to perform a sensory discrimination task involving barrel cortex

Depletion of cortical acetylcholine (ACh) correlates with reduced stimulus-evoked 2-deoxyglucose (2-DG) uptake in rodent somatosensory cortex. We examined the effect of unilateral basal forebrain lesions and subsequent cortical ACh depletion on (1) the ability of rats to detect a passively applied deflection of the whiskers, and (2) whisker-evoked 2-DG uptake. Normal adult rats were trained on a T-maze to respond by turning in one direction if the whiskers were displaced and in the opposite direction on presentation of a sham stimulus; only the left set of whiskers was stimulated. When the animal performed at the 80% correct level for three consecutive sessions (criterion), it was randomly assigned to a group receiving either a saline injection (sham lesion) or an ibotenic acid injection (excitotoxic lesion) into the right basal forebrain. Behavioral testing continued until the animal returned to the prelesion criterion, at which time a terminal 2-DG experiment was conducted. None of the sham-lesioned rats experienced disruption of their ability to perform the task. All excitotoxic basal forebrain-lesioned rats were impaired in task performance, but eventually returned to prelesion performance levels. The length of time required to return to criterion was positively correlated with the amount of cortical ACh depletion. Despite the behavioral recovery of the ACh-depleted rats, 2-DG uptake in response to whisker stimulation continued to be reduced in the somatosensory cortex ipsilateral to the basal forebrain lesion. These findings suggest that ACh depletion, which results in a long-lasting decrease in neuronal responses to evoked stimuli, transiently impairs an animal's ability to perceive and appropriately respond to sensory information; the duration of impairment is related to the degree of depletion.

Long-term delayed vascularization of human neural transplants to the rat brain

Human neural transplants are being developed to treat Parkinson's disease. Previous characterization of human transplants focused on neuronal development, while little is known of the interaction between the transplant and its environment, among which blood is of prime importance. We evaluated here the formation of blood vessels in human neural xenografts placed into the brain of rats immunosuppressed with cyclosporin A. Using capillary wall markers, we found that human transplants remain virtually nonvascularized for more than 1 month. Angiogenesis takes place very slowly and the density of blood vessels is still quite poor after 3 months, the fine structure of these capillaries, when they form, is apparently normal. Functional studies indicate that the vascular network formed in the transplant allows blood circulation and exhibits a working barrier to macromolecules. Glucose uptake and consumption and cytochrome oxidase activity are almost undetectable up to 3 months after grafting. These results demonstrate that vascularization is much delayed in human xenografts into the rat brain. This delay is likely to be dependent on the maturation of the transplanted tissue. A dedifferentiation of human endothelial cells cotransplanted with neural cells occurs since histochemical and immunocytochemical markers revealing endothelial cells in the human fetus are not present up to 1 month in the transplant. The origin of this phenomenon is a matter of speculation. How neural cells survive and mature in such conditions are issues of prime interest for the future of human neural grafting.