The diffusion of neuroactive substances in the extracellular space (ECS) plays an important role in short- and long-distance communication between nerve cells and is the underlying mechanism of extrasynaptic (volume) transmission. The diffusion properties of the ECS are described by three parameters: 1. ECS volume fraction alpha (alpha=ECS volume/total tissue volume), 2. tortuosity lambda (lambda2=free/apparent diffusion coefficient), reflecting the presence of diffusion barriers represented by, e.g., fine neuronal and glial processes or extracellular matrix molecules and 3. nonspecific uptake k'. These diffusion parameters differ in various brain regions, and diffusion in the CNS is therefore inhomogeneous. Moreover, diffusion barriers may channel the migration of molecules in the ECS, so that diffusion is facilitated in a certain direction, i.e. diffusion in certain brain regions is anisotropic. Changes in the diffusion parameters have been found in many physiological and pathological states 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 in the CNS and thus extrasynaptic transmission, neuron-glia communication, transmitter "spillover" and synaptic cross-talk as well as cell migration, drug delivery and treatment.

• MeSH
• anizotropie MeSH
• centrální nervový systém fyziologie   MeSH
• difuze MeSH
• extracelulární prostor fyziologie   MeSH
• krysa rodu rattus MeSH
• lékové transportní systémy MeSH
• lidé MeSH
• myši transgenní MeSH
• myši MeSH
• neuroglie fyziologie   MeSH
• neurony fyziologie   MeSH
• pohyb buněk fyziologie   MeSH
• signální transdukce fyziologie   MeSH
• zvířata MeSH
• Check Tag
• krysa rodu rattus MeSH
• lidé MeSH
• myši MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• přehledy MeSH

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.

• MeSH
• difuze MeSH
• extracelulární prostor MeSH
• financování organizované MeSH
• lidé MeSH
• nervový přenos MeSH
• neuroglie fyziologie   MeSH
• zvířata MeSH
• Check Tag
• lidé MeSH
• zvířata MeSH
• Publikační typ
• přehledy MeSH

Focal cortical dysplasias (FCDs) of the brain are recognized as a frequent cause of intractable epilepsy. To contribute to the current understanding of the mechanisms of epileptogenesis in FCD, our study provides evidence that not only cellular alterations and synaptic transmission, but also changed diffusion properties of the extracellular space (ECS), induced by modified extracellular matrix (ECM) composition and astrogliosis, might be involved in the generation or spread of seizures in FCD. The composition of the ECM in FCD and non-malformed cortex (in 163 samples from 62 patients) was analyzed immunohistochemically and correlated with the corresponding ECS diffusion parameter values determined with the real-time iontophoretic method in freshly resected cortex (i.e. the ECS volume fraction and the geometrical factor tortuosity, describing the hindrances to diffusion in the ECS). The ECS in FCD was shown to differ from that in non-malformed cortex, mainly by the increased accumulation of certain ECM molecules (tenascin R, tenascin C, and versican) or by their reduced expression (brevican), and by the presence of an increased number of astrocytic processes. The consequent increase of ECS diffusion barriers observed in both FCD type I and II (and, at the same time, the enlargement of the ECS volume in FCD type II) may alter the diffusion of neuroactive substances through the ECS, which mediates one of the important modes of intercellular communication in the brain - extrasynaptic volume transmission. Thus, the changed ECM composition and altered ECS diffusion properties might represent additional factors contributing to epileptogenicity in FCD.

• MeSH
• astrocyty metabolismus   MeSH
• brevican analýza   MeSH
• difuze MeSH
• dítě MeSH
• dospělí MeSH
• extracelulární matrix chemie   metabolismus   MeSH
• extracelulární prostor chemie   metabolismus   MeSH
• iontoforéza metody   MeSH
• lidé středního věku MeSH
• lidé MeSH
• malformace mozkové kůry metabolismus   patologie   MeSH
• mladiství MeSH
• mladý dospělý MeSH
• nemoci mozku metabolismus   patologie   MeSH
• neokortex patologie   MeSH
• předškolní dítě MeSH
• tenascin analýza   MeSH
• versikany analýza   MeSH
• Check Tag
• dítě MeSH
• dospělí MeSH
• lidé středního věku MeSH
• lidé MeSH
• mladiství MeSH
• mladý dospělý MeSH
• mužské pohlaví MeSH
• předškolní dítě MeSH
• ženské pohlaví MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

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.

• MeSH
• iontoforéza MeSH
• lidé MeSH
• magnetická rezonanční tomografie metody   MeSH
• mozek fyziologie   MeSH
• zvířata MeSH
• Check Tag
• lidé MeSH
• zvířata MeSH
• Publikační typ
• práce podpořená grantem MeSH
• přehledy MeSH

The real-time iontophoretic method using tetramethylammonium-selective microelectrodes and diffusion-weighted magnetic resonance imaging were used to measure the extracellular space volume fraction alpha, tortuosity lambda and apparent diffusion coefficient of water (ADC(W)) 240 min after the administration of pilocarpine in urethane-anaesthetized rats. The obtained data were correlated with extracellular lactate, glucose, and glutamate concentrations and the lactate/pyruvate-ratio, determined by intracerebral microdialysis. The control values of alpha and lambda were 0.19+/-0.004 and 1.58+/-0.01, respectively. Following pilocarpine application, alpha decreased to 0.134+/-0.012 100 min later. Thereafter alpha increased, reaching 0.176+/-0.009 140 min later. No significant changes in lambda were observed during the entire time course of the experiment. ADC(W) was significantly decreased 100 min after pilocarpine application (549+/-8 microm(2) s(-1)) compared to controls (603+/-11 microm(2) s(-1)); by the end of the experiments, ADC(W) had returned to control values. The basal cortical levels of lactate, the lactate/pyruvate ratio, glucose and glutamate were 0.61+/-0.05 mmol/l, 33.16+/-4.26, 2.42+/-0.13 mmol/l and 6.55+/-1.31 micromol/l. Pilocarpine application led to a rise in lactate, the lactate/pyruvate ratio and glutamate levels, reaching 2.92+/-0.60 mmol/l, 84.80+/-11.72 and 22.39+/-5.85 micromol/l within about 100 min, with a subsequent decrease to control values 140 min later. The time course of changes in glucose levels was different, with maximal levels of 3.49+/-0.24 mmol/l reached 40 min after pilocarpine injection and a subsequent decrease to 1.25+/-0.40 mmol/l observed 200 min later. Pathologically increased neuronal activity induced by pilocarpine causes cell swelling followed by a reduction in the ECS volume fraction, which can contribute to the accumulation of toxic metabolites and lead to the start of epileptic discharges.

• MeSH
• agonisté muskarinových receptorů MeSH
• difuzní magnetická rezonance MeSH
• draslík metabolismus   MeSH
• elektrický šok MeSH
• energetický metabolismus fyziologie   účinky léků   fyziologie   MeSH
• extracelulární prostor metabolismus   účinky léků   MeSH
• financování organizované MeSH
• glukosa metabolismus   MeSH
• krysa rodu rattus MeSH
• kyselina mléčná MeSH
• membránové potenciály fyziologie   MeSH
• mikrodialýza MeSH
• mozek - chemie fyziologie   účinky léků   MeSH
• mozková kůra metabolismus   patofyziologie   MeSH
• pilokarpin MeSH
• potkani Wistar MeSH
• sodík metabolismus   MeSH
• status epilepticus chemicky indukované   metabolismus   patofyziologie   MeSH
• tělesná voda metabolismus   MeSH
• zvířata MeSH
• Check Tag
• krysa rodu rattus MeSH
• mužské pohlaví MeSH
• zvířata MeSH

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.

• MeSH
• difuze MeSH
• extracelulární prostor fyziologie   chemie   MeSH
• financování organizované MeSH
• kvartérní amoniové sloučeniny diagnostické užití   MeSH
• lidé MeSH
• mozek - chemie fyziologie   MeSH
• mozek cytologie   fyziologie   MeSH
• neuroglie fyziologie   MeSH
• neurony fyziologie   MeSH
• zvířata MeSH
• Check Tag
• lidé MeSH
• zvířata MeSH
• Publikační typ
• přehledy MeSH

Most hypotheses concerning the mechanisms underlying seizure activity in focal cortical dysplasia (FCD) are based on alterations in synaptic transmission and glial dysfunction. However, neurons may also communicate by extrasynaptic transmission, which was recently found to affect epileptiform activity under experimental conditions and which is mediated by the diffusion of neuroactive substances in the extracellular space (ECS). The ECS diffusion parameters were therefore determined using the real-time iontophoretic method in human neocortical tissue samples obtained from surgically treated epileptic patients. The obtained values of the extracellular space volume fraction and tortuosity were then correlated with the histologicaly assessed type of cortical malformation (FCD type I or II). While the extracellular volume remained unchanged (FCD I) or larger (FCD II) than in normal/control tissue, tortuosity was significantly increased in both types of dysplasia, indicating the presence of additional diffusion barriers and compromised diffusion, which might be another factor contributing to the epileptogenicity of FCD.

• MeSH
• dítě MeSH
• dospělí MeSH
• epilepsie patologie   MeSH
• extracelulární prostor fyziologie   MeSH
• lidé středního věku MeSH
• lidé MeSH
• malformace mozkové kůry patologie   MeSH
• mladiství MeSH
• mladý dospělý MeSH
• mozková kůra abnormality   patologie   MeSH
• neurony patologie   fyziologie   MeSH
• Check Tag
• dítě MeSH
• dospělí MeSH
• lidé středního věku MeSH
• lidé MeSH
• mladiství MeSH
• mladý dospělý MeSH
• mužské pohlaví MeSH
• ženské pohlaví MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

Changes in brain extracellular space (ECS) volume, composition, and geometry are a consequence of neuronal activity, of glial K+, pH, and amino acid homeostasis, and of changes in glial cell morphology, proliferation, and function. They occur as a result of repetitive neuronal activity, seizures, anoxia, injury, inflammation, and many other pathological states in the CNS, and may significantly affect signal transmission in the CNS. Activity-related or CNS damage-related cellular swelling is compensated for by ECS volume shrinkage and, as a consequence, by a decrease in the apparent diffusion coefficients (ADCs) of neuroactive substances diffusing in the ECS. Changes in cellular morphology, such as occur during aging, could also result in changes of ECS volume and geometry. We provide evidence for limited diffusion in rat cortex, corpus callosum, and hippocampus in the aging brain that correlates with changes in glial volume and the extracellular matrix. In all structures, the mean ECS volume fraction alpha (alpha = ECS volume/total tissue volume) and nonspecific uptake k' are significantly lower in aged rats (26-32 months old) than in young adult brain. Compared to young adult brain, in the aged brain we found an increase in GFAP staining and hypertrophied astrocytes with thicker processes which, in the hippocampus, lost their radial organization. The tortuosity (lambda = square root of D/ADC) was lower in the cortex and CA3 region. Immunohistochemical staining for fibronectin and chondroitin sulfate proteoglycans revealed a substantial decrease that could account for a decrease in diffusion barriers. Diffusion parameters alpha, lambda, and k' in the aging brain after cardiac arrest changed substantially faster than in the young adult brain, although the final values were not significantly different. This suggests that the smaller extracellular space during aging results in a greater susceptibility of the aging brain to anoxia/ischemia, apparently due to a faster extracellular acidosis and accumulation of K+ and toxic substances, for example, glutamate. We conclude that during aging the movement of substances is more hindered in the narrower clefts. This is partly compensated for by a decrease in the diffusion barriers that may be formed by macromolecules of the extracellular matrix. Diffusion parameters can affect the efficacy of synaptic as well as extrasynaptic transmission by a greater accumulation of substances, because they diffuse away from a source more slowly, or induce damage to nerve cells if these substances reach toxic concentrations. Diffusion parameters are also of importance in the "crosstalk" between synapses, which has been hypothesized to be of importance during LTP and LTD. We can, therefore, assume that the observed changes in ECS diffusion parameters during aging can contribute to functional deficits and memory loss.

• MeSH
• difuze MeSH
• extracelulární prostor metabolismus   MeSH
• ischemie mozku metabolismus   patologie   MeSH
• krysa rodu rattus MeSH
• mezibuněčná komunikace * fyziologie   MeSH
• mozek cytologie   fyziologie   metabolismus   patologie   MeSH
• neuroglie * fyziologie   MeSH
• neurony * fyziologie   MeSH
• potkani Wistar MeSH
• stárnutí * fyziologie   MeSH
• zvířata MeSH
• Check Tag
• krysa rodu rattus MeSH
• mužské pohlaví MeSH
• ženské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• práce podpořená grantem MeSH

Aquaporin-4 (AQP4) is the primary cellular water channel in the brain and is abundantly expressed by astrocytes along the blood-brain barrier and brain-cerebrospinal fluid interfaces. Water transport via AQP4 contributes to the activity-dependent volume changes of the extracellular space (ECS), which affect extracellular solute concentrations and neuronal excitability. AQP4 is anchored by α-syntrophin (α-syn), the deletion of which leads to reduced AQP4 levels in perivascular and subpial membranes. We used the real-time iontophoretic method and/or diffusion-weighted magnetic resonance imaging to clarify the impact of α-syn deletion on astrocyte morphology and changes in extracellular diffusion associated with cell swelling in vitro and in vivo. In mice lacking α-syn, we found higher resting values of the apparent diffusion coefficient of water (ADCW) and the extracellular volume fraction (α). No significant differences in tortuosity (λ) or non-specific uptake (k'), were found between α-syn-negative (α-syn -/-) and α-syn-positive (α-syn +/+) mice. The deletion of α-syn resulted in a significantly smaller relative decrease in α observed during elevated K(+) (10 mM) and severe hypotonic stress (-100 mOsmol/l), but not during mild hypotonic stress (-50 mOsmol/l). After the induction of terminal ischemia/anoxia, the final values of ADCW as well as of the ECS volume fraction α indicate milder cell swelling in α-syn -/- in comparison with α-syn +/+ mice. Shortly after terminal ischemia/anoxia induction, the onset of a steep rise in the extracellular potassium concentration and an increase in λ was faster in α-syn -/- mice, but the final values did not differ between α-syn -/- and α-syn +/+ mice. This study reveals that water transport through AQP4 channels enhances and accelerates astrocyte swelling. The substantially altered ECS diffusion parameters will likely affect the movement of neuroactive substances and/or trophic factors, which in turn may modulate the extent of tissue damage and/or drug distribution.

• MeSH
• akvaporin 4 metabolismus   MeSH
• astrocyty metabolismus   MeSH
• delece genu * MeSH
• difuze MeSH
• draslík metabolismus   MeSH
• extracelulární prostor metabolismus   MeSH
• genotyp MeSH
• genový knockout MeSH
• ischemie genetika   MeSH
• membránové proteiny genetika   metabolismus   MeSH
• myši knockoutované MeSH
• myši MeSH
• osmotický tlak MeSH
• proteiny vázající vápník genetika   metabolismus   MeSH
• somatosenzorické korové centrum metabolismus   MeSH
• srdeční zástava genetika   metabolismus   MeSH
• svalové proteiny genetika   metabolismus   MeSH
• zvířata MeSH
• Check Tag
• mužské pohlaví MeSH
• myši MeSH
• ženské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

It has been shown that global anoxia leads to dramatic changes in the diffusion properties of the extracellular space (ECS). In this study, we investigated how changes in ECS volume and geometry in the rat somatosensory cortex during and after transient hypoxia/ischemia correlate with extracellular concentrations of energy-related metabolites and glutamate. Adult male Wistar rats (n = 12) were anesthetized and subjected to hypoxia/ischemia for 30 min (ventilation with 10% oxygen and unilateral carotid artery occlusion). The ECS diffusion parameters, volume fraction and tortuosity, were determined from concentration-time profiles of tetramethylammonium applied by iontophoresis. Concentrations of lactate, glucose, pyruvate and glutamate in the extracellular fluid (ECF) were monitored by microdialysis (n = 9). During hypoxia/ischemia, the ECS volume fraction decreased from initial values of 0.19 +/- 0.03 (mean +/- S.E.M.) to 0.07 +/- 0.01 and tortuosity increased from 1.57 +/- 0.01 to 1.88 +/- 0.03. During reperfusion the volume fraction returned to control values within 20 min and then increased to 0.23 +/- 0.01, while tortuosity only returned to original values (1.53 +/- 0.06). The concentrations of lactate and glutamate, and the lactate/pyruvate ratio, substantially increased during hypoxia/ischemia, followed by continuous recovery during reperfusion. The glucose concentration decreased rapidly during hypoxia/ischemia with a subsequent return to control values within 20 min of reperfusion. We conclude that transient hypoxia/ischemia causes similar changes in ECS diffusion parameters as does global anoxia and that the time course of the reduction in ECS volume fraction correlates with the increase of extracellular concentration of glutamate. The decrease in the ECS volume fraction can therefore contribute to an increased accumulation of toxic metabolites, which may aggravate functional deficits and lead to damage of the central nervous system (CNS).

• MeSH
• energetický metabolismus MeSH
• extracelulární prostor metabolismus   MeSH
• glukosa metabolismus   MeSH
• kinetika MeSH
• krysa rodu rattus MeSH
• kyselina glutamová metabolismus   MeSH
• kyselina mléčná metabolismus   MeSH
• kyselina pyrohroznová metabolismus   MeSH
• mikrodialýza MeSH
• modely nemocí na zvířatech MeSH
• mozková hypoxie a ischemie metabolismus   patofyziologie   MeSH
• mozková kůra metabolismus   patofyziologie   MeSH
• potkani Wistar MeSH
• zvířata MeSH
• Check Tag
• krysa rodu rattus MeSH
• mužské pohlaví MeSH
• zvířata MeSH