Evolution of anisotropic diffusion in the developing rat corpus callosum

Lessons on brain edema in HE: from cellular to animal models and clinical studies

Brain edema is considered as a common feature associated with hepatic encephalopathy (HE). However, its central role as cause or consequence of HE and its implication in the development of the neurological alterations linked to HE are still under debate. It is now well accepted that type A and type C HE are biologically and clinically different, leading to different manifestations of brain edema. As a result, the findings on brain edema/swelling in type C HE are variable and sometimes controversial. In the light of the changing natural history of liver disease, better description of the clinical trajectory of cirrhosis and understanding of molecular mechanisms of HE, and the role of brain edema as a central component in the pathogenesis of HE is revisited in the current review. Furthermore, this review highlights the main techniques to measure brain edema and their advantages/disadvantages together with an in-depth description of the main ex-vivo/in-vivo findings using cell cultures, animal models and humans with HE. These findings are instrumental in elucidating the role of brain edema in HE and also in designing new multimodal studies by performing in-vivo combined with ex-vivo experiments for a better characterization of brain edema longitudinally and of its role in HE, especially in type C HE where water content changes are small.

Anesthesia-related brain microstructure modulations detected by diffusion magnetic resonance imaging

Recent studies have shown significant changes to brain microstructure during sleep and anesthesia. In vivo optical microscopy and magnetic resonance imaging (MRI) studies have attributed these changes to anesthesia and sleep-related modulation of the brain's extracellular space (ECS). Isoflurane anesthesia is widely used in preclinical diffusion MRI (dMRI) and it is therefore important to investigate if the brain's microstructure is affected by anesthesia to an extent detectable with dMRI. Here, we employ diffusion kurtosis imaging (DKI) to assess brain microstructure in the awake and anesthetized mouse brain (n = 22). We find both mean diffusivity (MD) and mean kurtosis (MK) to be significantly decreased in the anesthetized mouse brain compared with the awake state (p < 0.001 for both). This effect is observed in both gray matter and white matter. To further investigate the time course of these changes we introduce a method for time-resolved fast DKI. With this, we show the time course of the microstructural alterations in mice (n = 5) as they transition between states in an awake-anesthesia-awake paradigm. We find that the decrease in MD and MK occurs rapidly after delivery of gas isoflurane anesthesia and that values normalize only slowly when the animals return to the awake state. Finally, time-resolved fast DKI is employed in an experimental mouse model of brain edema (n = 4), where cell swelling causes the ECS volume to decrease. Our results show that isoflurane affects DKI parameters and metrics of brain microstructure and point to isoflurane causing a reduction in the ECS volume. The demonstrated DKI methods are suitable for in-bore perturbation studies, for example, for investigating microstructural modulations related to sleep/wake-dependent functions of the glymphatic system. Importantly, our study shows an effect of isoflurane anesthesia on rodent brain microstructure that has broad relevance to preclinical dMRI.

Understanding Neuropeptide Transmission in the Brain by Optical Uncaging and Release

Neuropeptides are abundant and essential signaling molecules in the nervous system involved in modulating neural circuits and behavior. Neuropeptides are generally released extrasynaptically and signal via volume transmission through G-protein-coupled receptors (GPCR). Although substantive functional roles of neuropeptides have been discovered, many questions on neuropeptide transmission remain poorly understood, including the local diffusion and transmission properties in the brain extracellular space. To address this challenge, intensive efforts are required to develop advanced tools for releasing and detecting neuropeptides with high spatiotemporal resolution. Because of the rapid development of biosensors and materials science, emerging tools are beginning to provide a better understanding of neuropeptide transmission. In this perspective, we summarize the fundamental advances in understanding neuropeptide transmission over the past decade, highlight the tools for releasing neuropeptides with high spatiotemporal solution in the brain, and discuss open questions and future directions in the field.

Local diffusion in the extracellular space of the brain

The brain extracellular space (ECS) is a vast interstitial reticulum of extreme morphological complexity, composed of narrow gaps separated by local expansions, enabling interconnected highways between neural cells. Constituting on average 20% of brain volume, the ECS is key for intercellular communication, and understanding its diffusional properties is of paramount importance for understanding the brain. Within the ECS, neuroactive substances travel predominantly by diffusion, spreading through the interstitial fluid and the extracellular matrix scaffold after being focally released. The nanoscale dimensions of the ECS render it unresolvable by conventional live tissue compatible imaging methods, and historically diffusion of tracers has been used to indirectly infer its structure. Novel nanoscopic imaging techniques now show that the ECS is a highly dynamic compartment, and that diffusivity in the ECS is more heterogeneous than anticipated, with great variability across brain regions and physiological states. Diffusion is defined primarily by the local ECS geometry, and secondarily by the viscosity of the interstitial fluid, including the obstructive and binding properties of the extracellular matrix. ECS volume fraction and tortuosity both strongly determine diffusivity, and each can be independently regulated e.g. through alterations in glial morphology and the extracellular matrix composition. Here we aim to provide an overview of our current understanding of the ECS and its diffusional properties. We highlight emerging technological advances to respectively interrogate and model diffusion through the ECS, and point out how these may contribute in resolving the remaining enigmas of the ECS.

Convection-Enhanced Delivery and Principles of Extracellular Transport in the Brain

Stereotactic neurosurgery involves a targeted intervention based on congruence of image guidance to a reference fiducial system. This discipline has widespread applications in radiosurgery, tumor therapy, drug delivery, functional lesioning, and neuromodulation. In this article, we focused on convection-enhanced delivery to deliver therapeutic agents to the brain addressing areas of research and clinical development. We performed a robust literature review of all relevant articles highlighting current efforts and challenges of making this delivery technique more widely understood. We further described key biophysical properties of molecular transport in the extracellular space that may impact the efficacy and control of drug delivery using stereotactic methods. Understanding these principles is critical for further refinement of predictive models that can inform advances in stereotactic techniques for convection-enhanced delivery of therapeutic agents to the brain.

Characterizing extracellular diffusion properties using diffusion-weighted MRS of sucrose injected in mouse brain

Brain water and some critically important energy metabolites, such as lactate or glucose, are present in both intracellular and extracellular spaces (ICS/ECS) at significant levels. This ubiquitous nature makes diffusion MRI/MRS data sometimes difficult to interpret and model. While it is possible to glean information on the diffusion properties in ICS by measuring the diffusion of purely intracellular endogenous metabolites (such as NAA), the absence of endogenous markers specific to ECS hampers similar analyses in this compartment. In past experiments, exogenous probes have therefore been injected into the brain to assess their apparent diffusion coefficient (ADC) and thus estimate tortuosity in ECS. Here, we use a similar approach in mice by injecting sucrose, a well-known ECS marker, in either the lateral ventricles or directly in the prefrontal cortex. For the first time, we propose a thorough characterization of ECS diffusion properties encompassing (1) short-range restriction by looking at signal attenuation at high b values, (2) tortuosity and long-range restriction by measuring ADC time-dependence at long diffusion times and (3) microscopic anisotropy by performing double diffusion encoding (DDE) measurements. Overall, sucrose diffusion behavior is strikingly different from that of intracellular metabolites. Acquisitions at high b values not only reveal faster sucrose diffusion but also some sensitivity to restriction, suggesting that the diffusion in ECS is not fully Gaussian at high b. The time evolution of the ADC at long diffusion times shows that the tortuosity regime is not reached yet in the case of sucrose, while DDE experiments suggest that it is not trapped in elongated structures. No major difference in sucrose diffusion properties is reported between the two investigated routes of injection and brain regions. These original experimental insights should be useful to better interpret and model the diffusion signal of molecules that are distributed between ICS and ECS compartments.

Current Techniques for Investigating the Brain Extracellular Space

The brain extracellular space (ECS) is a continuous reticular compartment that lies between the cells of the brain. It is vast in extent relative to its resident cells, yet, at the same time the nano- to micrometer dimensions of its channels and reservoirs are commonly finer than the smallest cellular structures. Our conventional view of this compartment as largely static and of secondary importance for brain function is rapidly changing, and its active dynamic roles in signaling and metabolite clearance have come to the fore. It is further emerging that ECS microarchitecture is highly heterogeneous and dynamic and that ECS geometry and diffusional properties directly modulate local diffusional transport, down to the nanoscale around individual synapses. The ECS can therefore be considered an extremely complex and diverse compartment, where numerous physiological events are unfolding in parallel on spatial and temporal scales that span orders of magnitude, from milliseconds to hours, and from nanometers to centimeters. To further understand the physiological roles of the ECS and identify new ones, researchers can choose from a wide array of experimental techniques, which differ greatly in their applicability to a given sample and the type of data they produce. Here, we aim to provide a basic introduction to the available experimental techniques that have been applied to address the brain ECS, highlighting their main characteristics. We include current gold-standard techniques, as well as emerging cutting-edge modalities based on recent super-resolution microscopy. It is clear that each technique comes with unique strengths and limitations and that no single experimental method can unravel the unknown physiological roles of the brain ECS on its own.

Integrity of White Matter is Compromised in Mice with Hyaluronan Deficiency

Brain white matter is the means of efficient signal propagation in brain and its dysfunction is associated with many neurological disorders. We studied the effect of hyaluronan deficiency on the integrity of myelin in murine corpus callosum. Conditional knockout mice lacking the hyaluronan synthase 2 were compared with control mice. Ultrastructural analysis by electron microscopy revealed a higher proportion of myelin lamellae intruding into axons of knockout mice, along with significantly slimmer axons (excluding myelin sheath thickness), lower g-ratios, and frequent loosening of the myelin wrappings, even though the myelin thickness was similar across the genotypes. Analysis of extracellular diffusion of a small marker molecule tetramethylammonium (74 MW) in brain slices prepared from corpus callosum showed that the extracellular space volume increased significantly in the knockout animals. Despite this vastly enlarged volume, extracellular diffusion rates were significantly reduced, indicating that the compromised myelin wrappings expose more complex geometric structure than the healthy ones. This finding was confirmed in vivo by diffusion-weighted magnetic resonance imaging. Magnetic resonance spectroscopy suggested that water was released from within the myelin sheaths. Our results indicate that hyaluronan is essential for the correct formation of tight myelin wrappings around the axons in white matter.

Analysis of convective and diffusive transport in the brain interstitium

Background

Despite advances in in vivo imaging and experimental techniques, the nature of transport mechanisms in the brain remain elusive. Mathematical modelling verified using available experimental data offers a powerful tool for investigating hypotheses regarding extracellular transport of molecules in brain tissue. Here we describe a tool developed to aid in investigation of interstitial transport mechanisms, especially the potential for convection (or bulk flow) and its relevance to interstitial solute transport, for which there is conflicting evidence.

Methods

In this work, we compare a large body of published experimental data for transport in the brain to simulations of purely diffusive transport and simulations of combined convective and diffusive transport in the brain interstitium, incorporating current theories of perivascular influx and efflux.

Results

The simulations show (1) convective flow in the interstitium potentially of a similar magnitude to diffusive transport for molecules of interest and (2) exchange between the interstitium and perivascular space, whereby fluid and solutes may enter or exit the interstitium, are consistent with the experimental data. Simulations provide an upper limit for superficial convective velocity magnitude (approximately \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$v$$\end{document}v = 50 μm min⁻¹), a useful finding for researchers developing techniques to measure interstitial bulk flow.

Conclusions

For the large molecules of interest in neuropathology, bulk flow may be an important mechanism of interstitial transport. Further work is warranted to investigate the potential for bulk flow.

Electronic supplementary material

The online version of this article (10.1186/s12987-019-0126-9) contains supplementary material, which is available to authorized users.

The Effect of iPS-Derived Neural Progenitors Seeded on Laminin-Coated pHEMA-MOETACl Hydrogel with Dual Porosity in a Rat Model of Chronic Spinal Cord Injury

Spinal cord injury (SCI), is a devastating condition leading to the loss of locomotor and sensory function below the injured segment. Despite some progress in acute SCI treatment using stem cells and biomaterials, chronic SCI remains to be addressed. We have assessed the use of laminin-coated hydrogel with dual porosity, seeded with induced pluripotent stem cell-derived neural progenitors (iPSC-NPs), in a rat model of chronic SCI. iPSC-NPs cultured for 3 weeks in hydrogel in vitro were positive for nestin, glial fibrillary acidic protein (GFAP) and microtubule-associated protein 2 (MAP2). These cell-polymer constructs were implanted into a balloon compression lesion, 5 weeks after lesion induction. Animals were behaviorally tested, and spinal cord tissue was immunohistochemically analyzed 28 weeks after SCI. The implanted iPSC-NPs survived in the scaffold for the entire experimental period. Host axons, astrocytes and blood vessels grew into the implant and an increased sprouting of host TH⁺ fibers was observed in the lesion vicinity. The implantation of iPSC-NP-LHM cell-polymer construct into the chronic SCI led to the integration of material into the injured spinal cord, reduced cavitation and supported the iPSC-NPs survival, but did not result in a statistically significant improvement of locomotor recovery.

Apparent diffusion coefficient changes in human brain during sleep - Does it inform on the existence of a glymphatic system?

The role of sleep in brain physiology is poorly understood. Recently rodent studies have shown that the glymphatic system clears waste products from brain more efficiently during sleep compared to wakefulness due to the expansion of the interstitial fluid space facilitating entry of cerebrospinal fluid (CSF) into the brain. Here, we studied water diffusivity in the brain during sleep and awake conditions, hypothesizing that an increase in water diffusivity during sleep would occur concomitantly with an expansion of CSF volume - an effect that we predicted based on preclinical findings would be most prominent in cerebellum. We used MRI to measure slow and fast components of the apparent diffusion coefficient (ADC) of water in the brain in 50 healthy participants, in 30 of whom we compared awake versus sleep conditions and in 20 of whom we compared rested-wakefulness versus wakefulness following one night of sleep-deprivation. Sleep compared to wakefulness was associated with increases in slow-ADC in cerebellum and left temporal pole and with decreases in fast-ADC in thalamus, insula, parahippocampus and striatal regions, and the density of sleep arousals was inversely associated with ADC changes. The CSF volume was also increased during sleep and was associated with sleep-induced changes in ADCs in cerebellum. There were no differences in ADCs with wakefulness following sleep deprivation compared to rested-wakefulness. Although we hypothesized increases in ADC with sleep, our findings uncovered both increases in slow ADC (mostly in cerebellum) as well as decreases in fast ADC, which could reflect the distinct biological significance of fast- and slow-ADC values in relation to sleep. While preliminary, our findings suggest a more complex sleep-related glymphatic function in the human brain compared to rodents. On the other hand, our findings of sleep-induced changes in CSF volume provide preliminary evidence that is consistent with a glymphatic transport process in the human brain.

Pharmacodynamics of the Glutamate Receptor Antagonists in the Rat Barrel Cortex

Epipial application is one of the approaches for drug delivery into the cortex. However, passive diffusion of epipially applied drugs through the cortical depth may be slow, and different drug concentrations may be achieved at different rates across the cortical depth. Here, we explored the pharmacodynamics of the inhibitory effects of epipially applied ionotropic glutamate receptor antagonists CNQX and dAPV on sensory-evoked and spontaneous activity across layers of the cortical barrel column in urethane-anesthetized rats. The inhibitory effects of CNQX and dAPV were observed at concentrations that were an order higher than in slices in vitro, and they slowly developed from the cortical surface to depth after epipial application. The level of the inhibitory effects also followed the surface-to-depth gradient, with full inhibition of sensory evoked potentials (SEPs) in the supragranular layers and L4 and only partial inhibition in L5 and L6. During epipial CNQX and dAPV application, spontaneous activity and the late component of multiple unit activity (MUA) during sensory-evoked responses were suppressed faster than the short-latency MUA component. Despite complete suppression of SEPs in L4, sensory-evoked short-latency multiunit responses in L4 persisted, and they were suppressed by further addition of lidocaine suggesting that spikes in thalamocortical axons contribute ∼20% to early multiunit responses. Epipial CNQX and dAPV also completely suppressed sensory-evoked very fast (∼500 Hz) oscillations and spontaneous slow wave activity in L2/3 and L4. However, delta oscillations persisted in L5/6. Thus, CNQX and dAPV exert inhibitory actions on cortical activity during epipial application at much higher concentrations than in vitro, and the pharmacodynamics of their inhibitory effects is characterized by the surface-to-depth gradients in the rate of development and the level of inhibition of sensory-evoked and spontaneous cortical activity.

A transcriptome-based assessment of the astrocytic dystrophin-associated complex in the developing human brain

Astrocytes play a critical role in regulating the interface between the cerebral vasculature and the central nervous system. Contributing to this is the astrocytic endfoot domain, a specialized structure that ensheathes the entirety of the vasculature and mediates signaling between endothelial cells, pericytes, and neurons. The astrocytic endfoot has been implicated as a critical element of the glymphatic pathway, and changes in protein expression profiles in this cellular domain are linked to Alzheimer's disease pathology. Despite this, basic physiological properties of this structure remain poorly understood including the developmental timing of its formation, and the protein components that localize there to mediate its functions. Here we use human transcriptome data from male and female subjects across several developmental stages and brain regions to characterize the gene expression profile of the dystrophin-associated complex (DAC), a known structural component of the astrocytic endfoot that supports perivascular localization of the astroglial water channel aquaporin-4. Transcriptomic profiling is also used to define genes exhibiting parallel expression profiles to DAC elements, generating a pool of candidate genes that encode gene products that may contribute to the physiological function of the perivascular astrocytic endfoot domain. We found that several genes encoding transporter proteins are transcriptionally associated with DAC genes.

The role of myelination in measures of white matter integrity: Combination of diffusion tensor imaging and two-photon microscopy of CLARITY intact brains

Diffusion tensor imaging (DTI) is used extensively in neuroscience to noninvasively estimate white matter (WM) microarchitecture. However, the diffusion signal is inherently ambiguous because it infers WM structure from the orientation of water diffusion and cannot identify the biological sources of diffusion changes. To compare inferred WM estimates to directly labeled axonal elements, we performed a novel within-subjects combination of high-resolution ex vivo DTI with two-photon laser microscopy of intact mouse brains rendered optically transparent by Clear Lipid-exchanged, Anatomically Rigid, Imaging/immunostaining compatible, Tissue hYdrogel (CLARITY). We found that myelin basic protein (MBP) immunofluorescence significantly correlated with fractional anisotropy (FA), especially in WM regions with coherent fiber orientations and low fiber dispersion. Our results provide evidence that FA is particularly sensitive to myelination in WM regions with these characteristics. Furthermore, we found that radial diffusivity (RD) was only sensitive to myelination in a subset of WM tracts, suggesting that the association of RD with myelin should be used cautiously. This combined DTI-CLARITY approach illustrates, for the first time, a framework for using brain-wide immunolabeling of WM targets to elucidate the relationship between the diffusion signal and its biological underpinnings. This study also demonstrates the feasibility of a within-subject combination of noninvasive neuroimaging and tissue clearing techniques that has broader implications for neuroscience research.

Astrocytes and extracellular matrix in extrasynaptic volume transmission

Volume transmission is a form of intercellular communication that does not require synapses; it is based on the diffusion of neuroactive substances across the brain extracellular space (ECS) and their binding to extrasynaptic high-affinity receptors on neurons or glia. Extracellular diffusion is restricted by the limited volume of the ECS, which is described by the ECS volume fraction α, and the presence of diffusion barriers, reflected by tortuosity λ, that are created, for example, by fine astrocytic processes or extracellular matrix (ECM) molecules. Organized astrocytic processes, ECM scaffolds or myelin sheets channel the extracellular diffusion so that it is facilitated in a certain direction, i.e. anisotropic. The diffusion properties of the ECS are profoundly influenced by various processes such as the swelling and morphological rebuilding of astrocytes during either transient or persisting physiological or pathological states, or the remodelling of the ECM in tumorous or epileptogenic tissue, during Alzheimer's disease, after enzymatic treatment or in transgenic animals. The changing diffusion properties of the ECM influence neuron-glia interaction, learning abilities, the extent of neuronal damage and even cell migration. From a clinical point of view, diffusion parameter changes occurring during pathological states could be important for diagnosis, drug delivery and treatment.

Influence of neuropathology on convection-enhanced delivery in the rat hippocampus

Local drug delivery techniques, such as convention-enhanced delivery (CED), are promising novel strategies for delivering therapeutic agents otherwise limited by systemic toxicity and blood-brain-barrier restrictions. CED uses positive pressure to deliver infusate homogeneously into interstitial space, but its distribution is dependent upon appropriate tissue targeting and underlying neuroarchitecture. To investigate effects of local tissue pathology and associated edema on infusate distribution, CED was applied to the hippocampi of rats that underwent electrically-induced, self-sustaining status epilepticus (SE), a prolonged seizure. Infusion occurred 24 hours post-SE, using a macromolecular tracer, the magnetic resonance (MR) contrast agent gadolinium chelated with diethylene triamine penta-acetic acid and covalently attached to albumin (Gd-albumin). High-resolution T1- and T2-relaxation-weighted MR images were acquired at 11.1 Tesla in vivo prior to infusion to generate baseline contrast enhancement images and visualize morphological changes, respectively. T1-weighted imaging was repeated post-infusion to visualize final contrast-agent distribution profiles. Histological analysis was performed following imaging to characterize injury. Infusions of Gd-albumin into injured hippocampi resulted in larger distribution volumes that correlated with increased injury severity, as measured by hyperintense regions seen in T2-weighted images and corresponding histological assessments of neuronal degeneration, myelin degradation, astrocytosis, and microglial activation. Edematous regions included the CA3 hippocampal subfield, ventral subiculum, piriform and entorhinal cortex, amygdalar nuclei, middle and laterodorsal/lateroposterior thalamic nuclei. This study demonstrates MR-visualized injury processes are reflective of cellular alterations that influence local distribution volume, and provides a quantitative basis for the planning of local therapeutic delivery strategies in pathological brain regions.

Experience-dependent plasticity in white matter microstructure: reasoning training alters structural connectivity

Diffusion tensor imaging (DTI) techniques have made it possible to investigate white matter plasticity in humans. Changes in DTI measures, principally increases in fractional anisotropy (FA), have been observed following training programs as diverse as juggling, meditation, and working memory. Here, we sought to test whether three months of reasoning training could alter white matter microstructure. We recruited participants (n = 23) who were enrolled in a course to prepare for the Law School Admission Test (LSAT), a test that places strong demands on reasoning skills, as well as age- and IQ-matched controls planning to take the LSAT in the future (n = 22). DTI data were collected at two scan sessions scheduled three months apart. In trained participants but not controls, we observed decreases in radial diffusivity (RD) in white matter connecting frontal cortices, and in mean diffusivity (MD) within frontal and parietal lobe white matter. Further, participants exhibiting larger gains on the LSAT exhibited greater decreases in MD in the right internal capsule. In summary, reasoning training altered multiple measures of white matter structure in young adults. While the cellular underpinnings are unknown, these results provide evidence of experience-dependent white matter changes that may not be limited to myelination.

A preclinical assessment of neural stem cells as delivery vehicles for anti-amyloid therapeutics

Transplantation of neural stems cells (NSCs) could be a useful means to deliver biologic therapeutics for late-stage Alzheimer's disease (AD). In this study, we conducted a small preclinical investigation of whether NSCs could be modified to express metalloproteinase 9 (MMP9), a secreted protease reported to degrade aggregated Aβ peptides that are the major constituents of the senile plaques. Our findings illuminated three issues with using NSCs as delivery vehicles for this particular application. First, transplanted NSCs generally failed to migrate to amyloid plaques, instead tending to colonize white matter tracts. Second, the final destination of these cells was highly influenced by how they were delivered. We found that our injection methods led to cells largely distributing to white matter tracts, which are anisotropic conduits for fluids that facilitate rapid distribution within the CNS. Third, with regard to MMP9 as a therapeutic to remove senile plaques, we observed high concentrations of endogenous metalloproteinases around amyloid plaques in the mouse models used for these preclinical tests with no evidence that the NSC-delivered enzymes elevated these activities or had any impact. Interestingly, MMP9-expressing NSCs formed substantially larger grafts. Overall, we observed long-term survival of NSCs in the brains of mice with high amyloid burden. Therefore, we conclude that such cells may have potential in therapeutic applications in AD but improved targeting of these cells to disease-specific lesions may be required to enhance efficacy.

Diffusion MRI at 25: exploring brain tissue structure and function

Diffusion MRI (or dMRI) came into existence in the mid-1980s. During the last 25 years, diffusion MRI has been extraordinarily successful (with more than 300,000 entries on Google Scholar for diffusion MRI). Its main clinical domain of application has been neurological disorders, especially for the management of patients with acute stroke. It is also rapidly becoming a standard for white matter disorders, as diffusion tensor imaging (DTI) can reveal abnormalities in white matter fiber structure and provide outstanding maps of brain connectivity. The ability to visualize anatomical connections between different parts of the brain, non-invasively and on an individual basis, has emerged as a major breakthrough for neurosciences. The driving force of dMRI is to monitor microscopic, natural displacements of water molecules that occur in brain tissues as part of the physical diffusion process. Water molecules are thus used as a probe that can reveal microscopic details about tissue architecture, either normal or in a diseased state.

From singing to speaking: facilitating recovery from nonfluent aphasia

It has been reported for more than 100 years that patients with severe nonfluent aphasia are better at singing lyrics than they are at speaking the same words. This observation led to the development of melodic intonation therapy (MIT). However, the efficacy of this therapy has yet to be substantiated in a randomized controlled trial. Furthermore, its underlying neural mechanisms remain unclear. The two unique components of MIT are the intonation of words and simple phrases using a melodic contour that follows the prosody of speech and the rhythmic tapping of the left hand that accompanies the production of each syllable and serves as a catalyst for fluency. Research has shown that both components are capable of engaging fronto-temporal regions in the right hemisphere, thereby making MIT particularly well suited for patients with large left hemisphere lesions who also suffer from nonfluent aphasia. Recovery from aphasia can happen in two ways: either through the recruitment of perilesional brain regions in the affected hemisphere, with variable recruitment of right-hemispheric regions if the lesion is small, or through the recruitment of homologous language and speech-motor regions in the unaffected hemisphere if the lesion of the affected hemisphere is extensive. Treatment-associated neural changes in patients undergoing MIT indicate that the unique engagement of right-hemispheric structures (e.g., the superior temporal lobe, primary sensorimotor, premotor and inferior frontal gyrus regions) and changes in the connections across these brain regions may be responsible for its therapeutic effect.

Bral1: its role in diffusion barrier formation and conduction velocity in the CNS

At the nodes of Ranvier, excitable axon membranes are exposed directly to the extracellular fluid. Cations are accumulated and depleted in the local extracellular nodal region during action potential propagation, but the impact of the extranodal micromilieu on signal propagation still remains unclear. Brain-specific hyaluronan-binding link protein, Bral1, colocalizes and forms complexes with negatively charged extracellular matrix (ECM) proteins, such as versican V2 and brevican, at the nodes of Ranvier in the myelinated white matter. The link protein family, including Bral1, appears to be the linchpin of these hyaluronan-bound ECM complexes. Here we report that the hyaluronan-associated ECM no longer shows a nodal pattern and that CNS nerve conduction is markedly decreased in Bral1-deficient mice even though there were no differences between wild-type and mutant mice in the clustering or transition of ion channels at the nodes or in the tissue morphology around the nodes of Ranvier. However, changes in the extracellular space diffusion parameters, measured by the real-time iontophoretic method and diffusion-weighted magnetic resonance imaging (MRI), suggest a reduction in the diffusion hindrances in the white matter of mutant mice. These findings provide a better understanding of the mechanisms underlying the accumulation of cations due to diffusion barriers around the nodes during saltatory conduction, which further implies the importance of the Bral1-based extramilieu for neuronal conductivity.

Growth of white matter in the adolescent brain: myelin or axon?

White matter occupies almost half of the human brain. It contains axons connecting spatially segregated modules and, as such, it is essential for the smooth flow of information in functional networks. Structural maturation of white matter continues during adolescence, as reflected in age-related changes in its volume, as well as in its microstructure. Here I review recent observations obtained with magnetic resonance imaging in typically developing adolescents and point out some of the known variations in structural properties of white matter vis-à-vis brain function in health and disease. I conclude by re-focusing the interpretations of MR-based studies of white matter from myelin to axon.

Mild hypoxic-ischemic injury in the neonatal rat brain: longitudinal evaluation of white matter using diffusion tensor MR imaging

BACKGROUND AND PURPOSE

Selective white matter (WM) damage is a known sequela of mild hypoxic-ischemic (HI) injury in the neonatal rat model. The aim of this study was to evaluate longitudinally mild HI-induced WM damage (represented by the external capsule [EC]) by diffusion tensor MR imaging (DTI) and to correlate the findings with histology.

MATERIALS AND METHODS

Seven-day-old Sprague-Dawley rats (n = 19) underwent unilateral ligation of the left common carotid artery followed by hypoxia for 50 minutes to create mild HI injury. DTI was performed longitudinally at 5 time points from day 1 to day 90 postinjury (n = 19, 16, 13, 11, 9, respectively), and fractional anisotropy (FA), trace, radial diffusivity (lambda( perpendicular)), and axial diffusivity (lambda(//)) of the injury and control contralateral ECs were quantified. Rats were randomly sacrificed (n = 15, in total), and the corresponding ECs were stained with hematoxylin-eosin, Luxol fast blue (LFB), and neurofilament (NF) to evaluate morphologic changes, amount of myelin, and axonal count at every time point. A paired t test was applied to evaluate statistical differences between both ECs, and the Pearson correlation test was used to evaluate the relationships between DTI indices and histologic evaluations. In addition, longitudinal changes in DTI indices and histologic evaluations were analyzed by a linear mixed model and an analysis of variance test, respectively.

RESULTS

We demonstrated significantly decreased FA, increased lambda( perpendicular), and similar lambda(//) in the injury compared with the control EC, which was persistent through all time points. Histologic evaluation by LFB and NF staining showed reduced myelin stain intensity in the injury EC and similar axonal counts in both ECs. Longitudinally, there was an increase in FA, a decrease in lambda( perpendicular) and trace, and stability in lambda(//) in both ECs. Also, there was progressive reduction in the differences in FA, trace, and lambda( perpendicular) between the injury and control EC, especially between day 1 and day 7 postinjury and in tandem with changes in myelin stain. FA was significantly correlated with myelin stain (r = 0.681, P < .01) and axonal count (r = 0.673, P < .01), whereas lambda( perpendicular) was significantly correlated with myelin stain only (r = -0.528, P < .01), and lambda(//), with axonal count only (r = 0.372, P = .043).

CONCLUSIONS

Diffusion indices can reflect dysmyelination in mild HI injury, continual myelination of both injury and control ECs with growth, and the partial recovery of myelin postinjury. We propose that diffusion indices may be used as biomarkers to monitor noninvasively the longitudinal changes of mild HI-induced WM damage.

Measuring diffusion parameters in the brain: comparing the real-time iontophoretic method and diffusion-weighted magnetic resonance

The extracellular space (ECS) diffusion parameters influence the movement of ions, neuroactive substances, hormones and metabolites in the nervous tissue. They also affect extrasynaptic transmission, a mode of signal transmission dependent solely on diffusion. This review compares in detail two methods for studying diffusion in the brain: the real-time iontophoretic tetramethylammonium method for ECS volume fraction and tortuosity measurements and diffusion weighted-magnetic resonance imaging for measuring the apparent diffusion coefficient of water. The results obtained using both methods under physiological conditions (post-natal development, ageing) or in pathologies (brain injury, ischaemia) and their similarities and differences are discussed.

Extracellular diffusion parameters in the rat somatosensory cortex during recovery from transient global ischemia/hypoxia

Changes in the extracellular space diffusion parameters during ischemia are well known, but information about changes during the postischemic period is lacking. Extracellular volume fraction (alpha) and tortuosity (lambda) were determined in the rat somatosensory cortex using the real-time iontophoretic method; diffusion-weighted magnetic resonance imaging was used to determine the apparent diffusion coefficient of water. Transient ischemia was induced by bilateral common carotid artery clamping for 10 or 15 mins and concomitant ventilation with 6% O(2) in N(2). In both ischemia groups, a negative DC shift accompanied by increased potassium levels occurred after 1 to 2 mins of ischemia and recovered to preischemic values within 3 to 5 mins of reperfusion. During ischemia of 10 mins duration, alpha typically decreased to 0.07+/-0.01, whereas lambda increased to 1.80+/-0.02. In this group, normal values of alpha=0.20+/-0.01 and lambda=1.55+/-0.01 were registered within 5 to 10 mins of reperfusion. After 15 mins of ischemia, alpha increased within 40 to 50 mins of reperfusion to 0.29+/-0.03 and remained at this level. Tortuosity (lambda) increased to 1.81+/-0.02 during ischemia, recovered within 5 to 10 mins of reperfusion, and was increased to 1.62+/-0.01 at the end of the experiment. The observed changes can affect the diffusion of ions, neurotransmitters, metabolic substances, and drugs in the nervous system.

Diffusion in brain extracellular space

Diffusion in the extracellular space (ECS) of the brain is constrained by the volume fraction and the tortuosity and a modified diffusion equation represents the transport behavior of many molecules in the brain. Deviations from the equation reveal loss of molecules across the blood-brain barrier, through cellular uptake, binding, or other mechanisms. Early diffusion measurements used radiolabeled sucrose and other tracers. Presently, the real-time iontophoresis (RTI) method is employed for small ions and the integrative optical imaging (IOI) method for fluorescent macromolecules, including dextrans or proteins. Theoretical models and simulations of the ECS have explored the influence of ECS geometry, effects of dead-space microdomains, extracellular matrix, and interaction of macromolecules with ECS channels. Extensive experimental studies with the RTI method employing the cation tetramethylammonium (TMA) in normal brain tissue show that the volume fraction of the ECS typically is approximately 20% and the tortuosity is approximately 1.6 (i.e., free diffusion coefficient of TMA is reduced by 2.6), although there are regional variations. These parameters change during development and aging. Diffusion properties have been characterized in several interventions, including brain stimulation, osmotic challenge, and knockout of extracellular matrix components. Measurements have also been made during ischemia, in models of Alzheimer's and Parkinson's diseases, and in human gliomas. Overall, these studies improve our conception of ECS structure and the roles of glia and extracellular matrix in modulating the ECS microenvironment. Knowledge of ECS diffusion properties is valuable in contexts ranging from understanding extrasynaptic volume transmission to the development of paradigms for drug delivery to the brain.

The effect of impermeable boundaries of arbitrary geometry on the apparent diffusion coefficient

The apparent diffusion coefficient (ADC) obtained from NMR measurements is modelled for diffusion in a compartment restricted by an impermeable boundary. For a given pulse sequence, the ADC can be determined from the connected velocity autocorrelation function (the second-order velocity cumulant), which we show can be expressed as a double surface integral over the boundary, involving the probability for molecules to diffuse from one boundary point to another. There is no restriction on the geometry of the boundary. This result allows a fast calculation of the ADC for an arbitrary time course of the diffusion-sensitizing gradient. Explicit examples are given for diffusion within three basic geometries for different pulse sequences. The ADCs measured with the Stejskal-Tanner pulse sequence and a more realistic pulse sequence with slice selection gradient and eddy current compensation are found to yield almost identical results. The application of the results are discussed in relation to determination of the microscopic structure of brain white matter.

Diffusion tensor quantification of the human midsagittal corpus callosum subdivisions across the lifespan

The midsagittal corpus callosum (CC) cross-sectional area subdivisions have been used as early and sensitive markers of human brain white matter connectivity, development, natural aging and disease. Despite the simplicity and conspicuity of the appearance of the CC on anatomical magnetic resonance imaging (MRI), the published quantitative MRI literature on its regional sex and age trajectories are contradictory. The availability of noninvasive quantitative methods to assess the CC regions across the human lifespan would help clarify its contribution to behavior and cognition. In this report, we extended the utility of a recently described semi-automated diffusion tensor imaging (DTI) tissue segmentation method to utilize the high orientation contrast of the CC on DTI. Using optimized DTI methods on a cohort of 121 right-handed children and adults aged 6-68 years, we examined the CC areas and corresponding DTI metrics of the different functionally specialized sectors of the CC. Both the area and fractional anisotropy metrics followed inverted U-shaped curves, while the mean and radial diffusivities followed U-curves reflecting white matter progressive and regressive myelination dynamics that continue into young adulthood.

Computational methods for predicting drug transport in anisotropic and heterogeneous brain tissue

Effective drug delivery for many neurodegenerative diseases or tumors of the central nervous system is challenging. Targeted invasive delivery of large macromolecules such as trophic factors to desired locations inside the brain is difficult due to anisotropy and heterogeneity of the brain tissue. Despite much experimental research, prediction of bio-transport phenomena inside the brain remains unreliable. This article proposes a rigorous computational approach for accurately predicting the fate of infused therapeutic agents inside the brain. Geometric and physiological properties of anisotropic and heterogeneous brain tissue affecting drug transport are accounted for by in-vivo diffusion tensor magnetic resonance imaging data. The three-dimensional brain anatomy is reconstructed accurately from subject-specific medical images. Tissue anisotropy and heterogeneity are quantified with the help of diffusion tensor imaging (DTI). Rigorous first principles physical transport phenomena are applied to predict the fate of a high molecular weight trophic factor infused into the midbrain. Computer prediction of drug distribution in humans accounting for heterogeneous and anisotropic brain tissue properties have not been adequately researched in open literature before.

Diffusion of flexible random-coil dextran polymers measured in anisotropic brain extracellular space by integrative optical imaging

There are a limited number of methods available to quantify the extracellular diffusion of macromolecules in an anisotropic brain region, e.g., an area containing numerous aligned fibers where diffusion is faster along the fibers than across. We applied the integrative optical imaging method to measure diffusion of the fluorophore Alexa Fluor 488 (molecular weight (MW) 547) and fluorophore-labeled flexible random-coil dextran polymers (dex3, MW 3000; dex75, MW 75,000; dex282, MW 282,000; dex525, MW 525,000) in the extracellular space (ECS) of the anisotropic molecular layer of the isolated turtle cerebellum. For all molecules, two-dimensional images acquired an elliptical shape with major and minor axes oriented along and across, respectively, the unmyelinated parallel fibers. The effective diffusion coefficients, D*(major) and D*(minor), decreased with molecular size. The diffusion anisotropy ratio (DAR = D*(major)/D*(minor)) increased for Alexa Fluor 488 through dex75 but then unexpectedly reached a plateau. We argue that dex282 and dex525 approach the ECS width and deform to diffuse. In support of this concept, scaling theory shows the diffusion behavior of dex282 and dex525 to be consistent with transition to a reptation regime, and estimates the average ECS width at approximately 31 nm. These findings have implications for the interstitial transport of molecules and drugs, and for modeling neurotransmitter diffusion during ectopic release and spillover.

Characterizing molecular probes for diffusion measurements in the brain

Brain diffusion properties are at present most commonly evaluated by magnetic resonance (MR) diffusion imaging. MR cannot easily distinguish between the extracellular and intracellular signal components, but the older technique of real-time iontophoresis (RTI) detects exclusively extracellular diffusion. Interpretation of the MR results would therefore benefit from auxiliary RTI measurements. This requires a molecular probe detectable by both techniques. Our aim was to specify a minimum set of requirements that such a diffusion probe should fulfill and apply it to two candidate probes: the cation tetramethylammonium (TMA(+)), used routinely in the RTI experiments, and the anion hexafluoroantimonate (SbF(6)(-)). Desirable characteristics of a molecular diffusion probe include predictable diffusion properties, stability, minimum interaction with cellular physiology, very slow penetration into the cells, and sufficiently strong and selective MR and RTI signals. These properties were evaluated using preparations of rat neocortical slices under normal and ischemic conditions, as well as solutions and agarose gel. While both molecules can be detected by MR and RTI, neither proved an ideal candidate. TMA(+) was very stable but it penetrated into the cells and accumulated there within tens of minutes. SbF(6)(-) did not enter the cells as readily but it was not stable, particularly in ischemic tissue and at higher temperatures. Its presence also resulted in a decreased extracellular volume. These probe properties help to interpret previously published MR data on TMA(+) diffusion and might play a role in other diffusion experiments obtained with them.

Extrasynaptic transmission and the diffusion parameters of the extracellular space

Extrasynaptic volume transmission, mediated by the diffusion of neuroactive substances in the extracellular space (ECS), plays an important role in short- and long-distance communication between nerve cells. The ability of a substance to reach extrasynaptic high-affinity receptors via diffusion depends on the ECS diffusion parameters, ECS volume fraction alpha (alpha=ECS volume/total tissue volume) and tortuosity lambda (lambda2=free/apparent diffusion coefficient), which reflects the presence of diffusion barriers represented by, e.g., fine astrocytic processes or extracellular matrix molecules. These barriers channel the migration of molecules in the ECS, so that diffusion may be facilitated in a certain direction, i.e. anisotropic. The diffusion parameters alpha and lambda differ in various brain regions, and diffusion in the CNS is therefore inhomogeneous. Changes in diffusion parameters have been found in many physiological and pathological states, such as development and aging, neuronal activity, lactation, ischemia, brain injury, degenerative diseases, tumor growth and others, in which cell swelling, glial remodeling and extracellular matrix changes are key factors influencing diffusion. Changes in ECS volume, tortuosity and anisotropy significantly affect the accumulation and diffusion of neuroactive substances and thus extrasynaptic transmission, neuron-glia communication, mediator "spillover" and synaptic crosstalk as well as, cell migration. The various changes occurring during pathological states can be important for diagnosis, drug delivery and treatment.

From the Golgi–Cajal mapping to the transmitter-based characterization of the neuronal networks leading to two modes of brain communication: Wiring and volume transmission

After Golgi-Cajal mapped neural circuits, the discovery and mapping of the central monoamine neurons opened up for a new understanding of interneuronal communication by indicating that another form of communication exists. For instance, it was found that dopamine may be released as a prolactin inhibitory factor from the median eminence, indicating an alternative mode of dopamine communication in the brain. Subsequently, the analysis of the locus coeruleus noradrenaline neurons demonstrated a novel type of lower brainstem neuron that monosynaptically and globally innervated the entire CNS. Furthermore, the ascending raphe serotonin neuron systems were found to globally innervate the forebrain with few synapses, and where deficits in serotonergic function appeared to play a major role in depression. We propose that serotonin reuptake inhibitors may produce antidepressant effects through increasing serotonergic neurotrophism in serotonin nerve cells and their targets by transactivation of receptor tyrosine kinases (RTK), involving direct or indirect receptor/RTK interactions. Early chemical neuroanatomical work on the monoamine neurons, involving primitive nervous systems and analysis of peptide neurons, indicated the existence of alternative modes of communication apart from synaptic transmission. In 1986, Agnati and Fuxe introduced the theory of two main types of intercellular communication in the brain: wiring and volume transmission (WT and VT). Synchronization of phasic activity in the monoamine cell clusters through electrotonic coupling and synaptic transmission (WT) enables optimal VT of monoamines in the target regions. Experimental work suggests an integration of WT and VT signals via receptor-receptor interactions, and a new theory of receptor-connexin interactions in electrical and mixed synapses is introduced. Consequently, a new model of brain function must be built, in which communication includes both WT and VT and receptor-receptor interactions in the integration of signals. This will lead to the unified execution of information handling and trophism for optimal brain function and survival.

Neurodevelopment of C57B/L6 mouse brain assessed by in vivo diffusion tensor imaging

Heterogeneous spatiotemporal patterns of C57B/L6 murine brain maturation during the first 7 weeks after birth (i.e. P15 to P45) were assessed in vivo by diffusion tensor imaging (DTI) at 9.4 T. Maps of apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were used to assess developmental changes. Because directionally encoded color (DEC) maps provide an efficient and straightforward way to visualize anisotropy direction, they were used to highlight the orientation-dominant anisotropic tissues. In the corpus callosum, the increases in FA (approximately 0.4 to approximately 0.6 from P15 to P45) were primarily dominant in the medial-lateral direction, whereas the ADC decreased slightly (approximately 0.8 x 10(-3) to approximately 0.5 x 10(-3) mm(2)/s from P15 to P45). Similar increases in FA (approximately 0.3 to approximately 0.4 from P15 to P45) and decreases in ADC (approximately 0.8 x 10(-3) to approximately 0.5 x 10(-3) mm(2)/s from P15 to P45) were found in the cingulate, but these anisotropic changes were dominant in the anterior-posterior direction. In the caudate putamen, there were significant FA increases (approximately 0.1 to approximately 0.2 from P15 to P45) dominant in the dorsal-ventral and anterior-posterior directions, whereas the ADC increased rapidly early in development (approximately 0.3 x 10(-3) to approximately 0.7 x 10(-3) mm(2)/s from P15 to P17). There were no significant changes in tissue anisotropy in the somatosensory regions (whisker, forelimb), but the ADC decreased slightly (approximately 0.7 x 10(-3) to approximately 0.5 x 10(-3) mm(2)/s from P15 to P45). Although the major differences in DEC values were mainly observed in white matter pathways, other cortical and subcortical regions showed some potential morphological changes that were consistent with classical histological findings. In summary, these results show that high-resolution DTI at high magnetic fields allows detection and quantification of brain structures throughout normal development in C57B/L6 mice in vivo.

Deriving the glutamate clearance time course from transporter currents in CA1 hippocampal astrocytes: transmitter uptake gets faster during development

At many excitatory synapses, the neurotransmitter glutamate diffuses beyond the synaptic cleft to activate extrasynaptic targets. The extent and impact of such transmitter "spillover" on the processing capacity of neuronal networks are unclear, in part because it remains unknown how far transmitter diffuses from its point of release before being removed from the extracellular space by high-affinity glutamate transporters. Synaptically activated, transporter-mediated currents (STCs) recorded in hippocampal astrocytes provide an experimental measure of glutamate uptake, but the time course of the STC may be shaped, or "filtered," by other factors and therefore not represent a direct indication of clearance rate. Here, STCs were recorded from astrocytes in rat hippocampal slices under conditions in which uptake capacity was reduced and the STC decay reflected a slowed rate of glutamate clearance. The temporal characteristics of the filtering mechanisms were extracted from these responses, and the glutamate clearance time course in control conditions was derived. The results indicate that glutamate can be cleared from the extrasynaptic space within 1 ms. Clearance is fastest in adult neuropil, corresponding to a developmental increase in glial transporter expression. Synaptically released glutamate is taken up at the same rate as glutamate released via flash photolysis, indicating that the spatial location of transporters relative to the site of glutamate release does not affect the time course of clearance. Slower clearance in young animals would permit glutamate to diffuse greater distances, indicating a particularly important role for extrasynaptic receptors early in development.

Developmental changes in diffusion anisotropy coincide with immature oligodendrocyte progression and maturation of compound action potential

Disruption of oligodendrocyte lineage progression is implicated in the white-matter injury that occurs in cerebral palsy. We have previously published a model in rabbits consistent with cerebral palsy. Little is known of normal white-matter development in perinatal rabbits. Using a multidimensional approach, we defined the relationship of oligodendrocyte lineage progression and functional maturation of axons to structural development of selected cerebral white-matter tracts as determined by diffusion tensor imaging (DTI). Immunohistochemical studies showed that late oligodendrocyte progenitors appear at gestational age 22 [embryonic day 22 (E22)], whereas immature oligodendrocytes appear at E25, and both increase rapidly with time (approximately 13 cells/mm2/d) until the onset of myelination. Myelination began at postnatal day 5 (P5) (E36) in the internal capsule (IC) and at P11 in the medial corpus callosum (CC), as determined by localization of sodium channels and myelin basic protein. DTI of the CC and IC showed that fractional anisotropy (FA) increased rapidly between E25 and P1 (E32) (11% per day) and plateaued (<5% per day) after the onset of myelination. Postnatal maturation of the compound action potential (CAP) showed a developmental pattern similar to FA, with a rapid rise between E29 and P5 (in the CC, 18% per day) and a slower rise from P5 to P11 (in the CC, <5% per day). The development of immature oligodendrocytes after E29 coincides with changes in FA and CAP area in both the CC and IC. These findings suggest that developmental expansion of immature oligodendrocytes during the premyelination period may be important in defining structural and functional maturation of the white matter.

Reduced extracellular space in the brain of tenascin-R- and HNK-1-sulphotransferase deficient mice

Tenascin-R (TN-R), a large extracellular glycoprotein, is an important component of the adult brain's extracellular matrix (ECM); tenascin-C (TN-C) is expressed mainly during early development, while human natural killer 1 (HNK-1) is a sulphated carbohydrate epitope that attaches to these molecules, modifying their adhesive properties. To assess their influence on extracellular space (ECS) volume and geometry, we used the real-time iontophoretic method to measure ECS volume fraction alpha and tortuosity lambda, and diffusion-weighted magnetic resonance imaging (MRI) to measure the apparent diffusion coefficient of water (ADC(W)). Measurements were performed in vivo in the cortex and CA1 hippocampal region of TN-R-, TN-C- and HNK-1 sulphotransferase (ST)-deficient adult mice and their wild-type littermate controls. In both cortex and hippocampus, the lack of TN-R or HNK-1 sulphotransferase resulted in a significant decrease in alpha and lambda. Compared with controls, alpha in TN-R-/- and ST-/- mice decreased by 22-26% and 9-15%, respectively. MRI measurements revealed a decreased ADC(W) in the cortex, hippocampus and thalamus. ADC(W) reflected the changes in alpha; the decrease in lambda indicated fewer diffusion obstacles in the ECS, presumably due to a decreased macromolecular content. No significant changes were found in TN-C-/- animals. We conclude that in TN-R-/- and ST-/- mice, which show morphological, electrophysiological and behavioural abnormalities, the ECS is reduced and its geometry altered. TN-R, as an important component of the ECM, appears to maintain an optimal distance between cells. The altered diffusion of neuroactive substances in the brain will inevitably affect extrasynaptic transmission, neuron-glia interactions and synaptic efficacy.

Extracellular space diffusion in central nervous system: anisotropic diffusion measured by elliptical surface photobleaching

Diffusion in the extracellular space (ECS) is crucial for normal central nervous system physiology. The determinants of ECS diffusion include viscous interactions with extracellular matrix/plasma membranes ("viscosity") and ECS geometry ("tortuosity"). To resolve viscosity versus tortuosity effects, we measured direction-dependent (anisotropic) diffusion in ECS in mouse spinal cord by photobleaching using an elliptical spot produced by a cylindrical lens in the excitation path. Anisotropic diffusion slowed fluorescence recovery when the long axis of the ellipse was parallel versus perpendicular to the direction of faster diffusion. A mathematical model was constructed to deduce diffusion coefficients (D(x), D(y)) from fluorescence recovery measured for parallel and perpendicular orientations of the long axis of the ellipse. Elliptical spot photobleaching was validated by photobleaching aqueous-phase fluorophores on a diffraction grating, where diffusion is one-dimensional. Measurement of the diffusion of 70 kDa FITC-dextran in spinal cord in living mice indicated that viscosity slows diffusion by approximately 1.8-fold compared with its diffusion in solution. ECS geometry hinders diffusion across (but not along) axonal fibers in spinal cord further by approximately fivefold. In cerebral cortex, however, approximately 50% of the hindrance to ECS diffusion comes from viscosity and approximately 50% from tortuosity. We suggest that the extracellular matrix might have evolved to facilitate rather than hinder diffusion even for large molecules.

Extrasynaptic volume transmission and diffusion parameters of the extracellular space

Extrasynaptic communication between neurons or neurons and glia is mediated by the diffusion of neuroactive substances in the volume of the extracellular space (ECS). The size and irregular geometry of the diffusion channels in the ECS substantially differ not only around individual cells but also in different CNS regions and thus affect and direct the movement of various neuroactive substances in the ECS. Diffusion in the CNS is therefore not only inhomogeneous, but often also anisotropic. The diffusion parameters in adult mammals (including humans), ECS volume fraction alpha (alpha=ECS volume/total tissue volume) and tortuosity lambda (lambda(2)=free/apparent diffusion coefficient), are typically 0.20-0.25 and 1.5-1.6, respectively, and as such hinder the diffusion of neuroactive substances and water. These diffusion parameters modulate neuronal signaling, neuron-glia communication and extrasynaptic "volume" transmission. A significant decrease in ECS volume fraction and an increase in diffusion barriers (tortuosity) occur during neuronal activity and pathological states. The changes are often related to cell swelling, cell loss, astrogliosis, the rearrangement of neuronal and astrocytic processes and changes in the extracellular matrix. They are also altered during physiological states such as development, lactation and aging. Plastic changes in ECS volume, tortuosity and anisotropy significantly affect neuron-glia communication, the spatial relation of glial processes toward synapses, glutamate or GABA "spillover" and synaptic crosstalk. The various changes in tissue diffusivity occurring during many pathological states are important for diagnosis, drug delivery and treatment.

Nonsynaptic communication in the central nervous system

Classical synaptic functions are important and suitable to relatively fast and discretely localized processes, but the nonclassical receptorial functions may be providing revolutionary possibilities for dealing at the cellular level with many of the more interesting and seemingly intractable features of neural and cerebral activities. Although different forms of nonsynaptic communication (volume transmission) often appear in different studies, their importance to modulate and mediate various functions is still not completely recognized. To establish the existence and the importance of nonsynaptic communication in the nervous system, here we cite pieces of evidence for each step of the interneuronal communication in the nonsynaptic context including the release into the extracellular space (ECS) and the extrasynaptic receptors and transporters that mediate nonsynaptic functions. We are now faced with a multiplicity of chemical communication. The fact that transmitters can even be released from nonsynaptic varicosities without being coupled to frequency-coded neuronal activity and they are able to diffuse over large distances indicates that there is a complementary mechanism of interneuronal communication to classical synaptic transmission. Nonconventional mediators that are also important part of the nonsynaptic world will also be overviewed.

Diffusion properties of the brain in health and disease

Extrasynaptic transmission between neurons and communication between neurons and glia are mediated by the diffusion of neuroactive substances in the extracellular space (ECS)--volume transmission. Diffusion in the CNS is inhomogeneous and often not uniform in all directions (anisotropic). Ionic changes and amino acid release result in cellular (particularly glial) swelling, compensated for by ECS shrinkage and a decrease in the apparent diffusion coefficients of neuroactive substances or water (ADCW). The diffusion parameters of the CNS in adult mammals (including humans), ECS volume fraction alpha (alpha = ECS volume/total tissue volume; normally 0.20-0.25) and tortuosity lambda (lambda2 = D/ADC; normally 1.5-1.6), hinder the diffusion of neuroactive substances and water. A significant decrease in ECS volume and an increase in diffusion barriers (tortuosity) and anisoptropy have been observed during stimulation, lactation or learning deficits during aging, due to structural changes such as astrogliosis, the re-arrangement of astrocytic processes and a loss of extracellular matrix. Decreases in the apparent diffusion coefficient of tetramethylammonium (ADCTMA) and ADCW due to astrogliosis and increased proteoglycan expression were found in the brain after injury and in grafts of fetal tissue. Tenascin-R and tenascin C-deficient mice also showed significant changes in ADCTMA and ADCW, suggesting an important role for extracellular matrix molecules in ECS diffusion. Changes in ECS volume, tortuosity and anisotropy significantly affect neuron-glia communication, the spatial relation of glial processes towards synapses, the efficacy of glutamate or GABA 'spillover' and synaptic crosstalk, the migration of cells, the action of hormones and the toxic effects of neuroactive substances and can be important for diagnosis, drug delivery and new treatment strategies.

THE ROLE OF VOLUME TRANSMISSION IN AN ENDOGENOUS BRAIN

Brain dynamics depends on synaptic, diffusive, and glial activities. Observations indicate that synaptic and diffusive activities modify each other's morphology, and glial activity modifies both. Synaptic activity modifies glial morphology. Whether diffusion modifies glial morphology has not been reported but it is reasonable to expect that it does. The relationship between these three transmission processes forms a closed-loop hierarchy of causation in brain dynamics, and the operation of that hierarchy may account for some of the seemingly bizarre properties of cognitive function.