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MRI-based assessment of function and dysfunction in myelinated axons. Proc Natl Acad Sci U S A 2018; 115:E10225-E10234. [PMID: 30297414 DOI: 10.1073/pnas.1801788115] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
Repetitive electrical activity produces microstructural alteration in myelinated axons, which may afford the opportunity to noninvasively monitor function of myelinated fibers in peripheral nervous system (PNS)/CNS pathways. Microstructural changes were assessed via two different magnetic-resonance-based approaches: diffusion fMRI and dynamic T2 spectroscopy in the ex vivo perfused bullfrog sciatic nerves. Using this robust, classical model as a platform for testing, we demonstrate that noninvasive diffusion fMRI, based on standard diffusion tensor imaging (DTI), can clearly localize the sites of axonal conduction blockage as might be encountered in neurotrauma or other lesion types. It is also shown that the diffusion fMRI response is graded in proportion to the total number of electrical impulses carried through a given locus. Dynamic T2 spectroscopy of the perfused frog nerves point to an electrical-activity-induced redistribution of tissue water and myelin structural changes. Diffusion basis spectrum imaging (DBSI) reveals a reversible shift of tissue water into a restricted isotropic diffusion signal component. Submyelinic vacuoles are observed in electron-microscopy images of tissue fixed during electrical stimulation. A slowing of the compound action potential conduction velocity accompanies repetitive electrical activity. Correlations between electrophysiology and MRI parameters during and immediately after stimulation are presented. Potential mechanisms and interpretations of these results are discussed.
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Pan WJ, Lee SY, Billings J, Nezafati M, Majeed W, Buckley E, Keilholz S. Detection of neural light-scattering activity in vivo: optical transmittance studies in the rat brain. Neuroimage 2018; 179:207-214. [PMID: 29908312 DOI: 10.1016/j.neuroimage.2018.06.039] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2018] [Revised: 05/28/2018] [Accepted: 06/11/2018] [Indexed: 01/11/2023] Open
Abstract
Optical studies of ex vivo brain slices where blood is absent show that neural activity is accompanied by significant intrinsic optical signals (IOS) related to activity-dependent scattering changes in neural tissue. However, the neural scattering signals have been largely ignored in vivo in widely-used IOS methods where absorption contrast from hemoglobin was employed. Changes in scattering were observed on a time scale of seconds in previous brain slice IOS studies, similar to the time scale for the hemodynamic response. Therefore, potential crosstalk between the scattering and absorption changes may not be ignored if they have comparable contributions to IOS. In vivo, the IOS changes linked to neural scattering have been elusive. To isolate neural scattering signals in vivo, we employed 2 implantable optodes for small-separation (2 mm) transmission measurements of local brain tissue in anesthetized rats. This unique geometry enables us to separate neuronal activity-related changes in neural tissue scattering from changes in blood absorption based upon the direction of the signal change. The changes in IOS scattering and absorption in response to up-states of spontaneous neuronal activity in cortical or subcortical structures have strong correlation to local field potentials, but significantly different response latencies. We conclude that activity-dependent neural tissue scattering in vivo may be an additional source of contrast for functional brain studies that provides complementary information to other optical or MR-based systems that are sensitive to hemodynamic contrast.
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Affiliation(s)
- Wen-Ju Pan
- Department of Biomedical Engineering, Emory University/Georgia Institute of Technology, 1760 Haygood Drive, HSRB W200, Atlanta, GA, 30322, USA.
| | - Seung Yup Lee
- Department of Biomedical Engineering, Emory University/Georgia Institute of Technology, 1760 Haygood Drive, HSRB W200, Atlanta, GA, 30322, USA
| | - Jacob Billings
- Department of Biomedical Engineering, Emory University/Georgia Institute of Technology, 1760 Haygood Drive, HSRB W200, Atlanta, GA, 30322, USA
| | - Maysam Nezafati
- Department of Biomedical Engineering, Emory University/Georgia Institute of Technology, 1760 Haygood Drive, HSRB W200, Atlanta, GA, 30322, USA
| | - Waqas Majeed
- Department of Biomedical Engineering, Emory University/Georgia Institute of Technology, 1760 Haygood Drive, HSRB W200, Atlanta, GA, 30322, USA
| | - Erin Buckley
- Department of Biomedical Engineering, Emory University/Georgia Institute of Technology, 1760 Haygood Drive, HSRB W200, Atlanta, GA, 30322, USA
| | - Shella Keilholz
- Department of Biomedical Engineering, Emory University/Georgia Institute of Technology, 1760 Haygood Drive, HSRB W200, Atlanta, GA, 30322, USA.
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Abe Y, Tsurugizawa T, Le Bihan D. Water diffusion closely reveals neural activity status in rat brain loci affected by anesthesia. PLoS Biol 2017; 15:e2001494. [PMID: 28406906 PMCID: PMC5390968 DOI: 10.1371/journal.pbio.2001494] [Citation(s) in RCA: 36] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/05/2016] [Accepted: 03/16/2017] [Indexed: 11/18/2022] Open
Abstract
Diffusion functional MRI (DfMRI) reveals neuronal activation even when neurovascular coupling is abolished, contrary to blood oxygenation level—dependent (BOLD) functional MRI (fMRI). Here, we show that the water apparent diffusion coefficient (ADC) derived from DfMRI increased in specific rat brain regions under anesthetic conditions, reflecting the decreased neuronal activity observed with local field potentials (LFPs), especially in regions involved in wakefulness. In contrast, BOLD signals showed nonspecific changes, reflecting systemic effects of the anesthesia on overall brain hemodynamics status. Electrical stimulation of the central medial thalamus nucleus (CM) exhibiting this anesthesia-induced ADC increase led the animals to transiently wake up. Infusion in the CM of furosemide, a specific neuronal swelling blocker, led the ADC to increase further locally, although LFP activity remained unchanged, and increased the current threshold awakening the animals under CM electrical stimulation. Oppositely, induction of cell swelling in the CM through infusion of a hypotonic solution (−80 milliosmole [mOsm] artificial cerebrospinal fluid [aCSF]) led to a local ADC decrease and a lower current threshold to wake up the animals. Strikingly, the local ADC changes produced by blocking or enhancing cell swelling in the CM were also mirrored remotely in areas functionally connected to the CM, such as the cingulate and somatosensory cortex. Together, those results strongly suggest that neuronal swelling is a significant mechanism underlying DfMRI. It has been reported that neuronal activation results in a decrease of water diffusion in activated neural tissue. This new approach, known as diffusion functional MRI (DfMRI), has high potential for functional imaging of the brain, as the currently widespread blood oxygenation level—dependent (BOLD)-functional MRI (fMRI) method, which is based on neurovascular coupling, remains an indirect marker of neuronal activation. Here, we show that the water apparent diffusion coefficient (ADC) derived from DfMRI increased in specific rat brain regions under anesthetic conditions, reflecting the decreased neuronal activity, especially in regions involved in wakefulness. Electrical stimulation of the central medial (CM) thalamic nucleus exhibiting this anesthesia-induced ADC increase led the animals to transiently wake up. Infusion of the CM with furosemide—a specific blocker of neuronal swelling—led the ADC to increase further locally and increased the current threshold for waking the animals. Conversely, induction of cell swelling in the CM through infusion of a hypotonic solution led to a local ADC decrease and a lower current threshold to wake the animals. Strikingly, the local ADC changes produced by blocking or enhancing cell swelling in the CM were also mirrored remotely in areas functionally connected to the CM, such as the cingulate and somatosensory cortex. Those results strongly suggest that neuronal swelling is a significant mechanism underlying DfMRI.
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Affiliation(s)
- Yoshifumi Abe
- NeuroSpin, Joliot Institute, Commissariat à l'énergie atomique et aux énergies alternatives, Gif-sur-Yvette, France
| | - Tomokazu Tsurugizawa
- NeuroSpin, Joliot Institute, Commissariat à l'énergie atomique et aux énergies alternatives, Gif-sur-Yvette, France
| | - Denis Le Bihan
- NeuroSpin, Joliot Institute, Commissariat à l'énergie atomique et aux énergies alternatives, Gif-sur-Yvette, France
- * E-mail:
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Akita T, Okada Y. Characteristics and roles of the volume-sensitive outwardly rectifying (VSOR) anion channel in the central nervous system. Neuroscience 2014; 275:211-31. [DOI: 10.1016/j.neuroscience.2014.06.015] [Citation(s) in RCA: 72] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2014] [Revised: 06/06/2014] [Accepted: 06/07/2014] [Indexed: 01/05/2023]
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5
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Qian T, Chen R, Nakamura M, Furukawa T, Kumada T, Akita T, Kilb W, Luhmann HJ, Nakahara D, Fukuda A. Activity-dependent endogenous taurine release facilitates excitatory neurotransmission in the neocortical marginal zone of neonatal rats. Front Cell Neurosci 2014; 8:33. [PMID: 24574969 PMCID: PMC3918584 DOI: 10.3389/fncel.2014.00033] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2013] [Accepted: 01/22/2014] [Indexed: 12/27/2022] Open
Abstract
In the developing cerebral cortex, the marginal zone (MZ), consisting of early-generated neurons such as Cajal-Retzius cells, plays an important role in cell migration and lamination. There is accumulating evidence of widespread excitatory neurotransmission mediated by γ-aminobutyric acid (GABA) in the MZ. Cajal-Retzius cells express not only GABAA receptors but also α2/β subunits of glycine receptors, and exhibit glycine receptor-mediated depolarization due to high [Cl−]i. However, the physiological roles of glycine receptors and their endogenous agonists during neurotransmission in the MZ are yet to be elucidated. To address this question, we performed optical imaging from the MZ using the voltage-sensitive dye JPW1114 on tangential neocortical slices of neonatal rats. A single electrical stimulus evoked an action-potential-dependent optical signal that spread radially over the MZ. The amplitude of the signal was not affected by glutamate receptor blockers, but was suppressed by either GABAA or glycine receptor antagonists. Combined application of both antagonists nearly abolished the signal. Inhibition of Na+, K+-2Cl− cotransporter by 20 µM bumetanide reduced the signal, indicating that this transporter contributes to excitation. Analysis of the interstitial fluid obtained by microdialysis from tangential neocortical slices with high-performance liquid chromatography revealed that GABA and taurine, but not glycine or glutamate, were released in the MZ in response to the electrical stimulation. The ambient release of taurine was reduced by the addition of a voltage-sensitive Na+ channel blocker. Immunohistochemistry and immunoelectron microscopy indicated that taurine was stored both in Cajal-Retzius and non-Cajal-Retzius cells in the MZ, but was not localized in presynaptic structures. Our results suggest that activity-dependent non-synaptic release of endogenous taurine facilitates excitatory neurotransmission through activation of glycine receptors in the MZ.
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Affiliation(s)
- Taizhe Qian
- Department of Neurophysiology, Hamamatsu University School of Medicine Hamamatsu, Japan
| | - Rongqing Chen
- Institute of Physiology, University Medical Center of the Johannes Gutenberg-University Mainz, Germany
| | - Masato Nakamura
- Department of Psychology and Behavioral Neuroscience, Hamamatsu University School of Medicine Hamamatsu, Japan
| | - Tomonori Furukawa
- Department of Neurophysiology, Hamamatsu University School of Medicine Hamamatsu, Japan
| | - Tatsuro Kumada
- Department of Neurophysiology, Hamamatsu University School of Medicine Hamamatsu, Japan ; Department of Occupational Therapy, Tokoha University Hamamatsu, Japan
| | - Tenpei Akita
- Department of Neurophysiology, Hamamatsu University School of Medicine Hamamatsu, Japan
| | - Werner Kilb
- Institute of Physiology, University Medical Center of the Johannes Gutenberg-University Mainz, Germany
| | - Heiko J Luhmann
- Institute of Physiology, University Medical Center of the Johannes Gutenberg-University Mainz, Germany
| | - Daiichiro Nakahara
- Department of Psychology and Behavioral Neuroscience, Hamamatsu University School of Medicine Hamamatsu, Japan
| | - Atsuo Fukuda
- Department of Neurophysiology, Hamamatsu University School of Medicine Hamamatsu, Japan
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6
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Water diffusion in brain cortex closely tracks underlying neuronal activity. Proc Natl Acad Sci U S A 2013; 110:11636-41. [PMID: 23801756 DOI: 10.1073/pnas.1303178110] [Citation(s) in RCA: 63] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Neuronal activity results in a local increase in blood flow. This concept serves as the basis for functional MRI. Still, this approach remains indirect and may fail in situations interfering with the neurovascular coupling mechanisms (drugs, anesthesia). Here we establish that water molecular diffusion is directly modulated by underlying neuronal activity using a rat forepaw stimulation model under different conditions of neuronal stimulation and neurovascular coupling. Under nitroprusside infusion, a neurovascular-coupling inhibitor, the diffusion response and local field potentials were maintained, whereas the hemodynamic response was abolished. As diffusion MRI reflects interactions of water molecules with obstacles (e.g., cell membranes), the observed changes point to a dynamic modulation of the neural tissue structure upon activation, which remains to be investigated. These findings represent a significant shift in concept from the current electrochemical and neurovascular coupling principles used for brain imaging, and open unique avenues to investigate mechanisms underlying brain function.
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Cesetti T, Ciccolini F, Li Y. GABA Not Only a Neurotransmitter: Osmotic Regulation by GABA(A)R Signaling. Front Cell Neurosci 2012; 6:3. [PMID: 22319472 PMCID: PMC3268181 DOI: 10.3389/fncel.2012.00003] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2011] [Accepted: 01/10/2012] [Indexed: 12/05/2022] Open
Abstract
Mature macroglia and almost all neural progenitor types express γ-aminobutyric (GABA) A receptors (GABAARs), whose activation by ambient or synaptic GABA, leads to influx or efflux of chloride (Cl−) depending on its electro-chemical gradient (ECl). Since the flux of Cl− is indissolubly associated to that of osmotically obliged water, GABAARs regulate water movements by modulating ion gradients. In addition, since water movements also occur through specialized water channels and transporters, GABAAR signaling could affect the movement of water by regulating the function of the channels and transporters involved, thereby affecting not only the direction of the water fluxes but also their dynamics. We will here review recent observations indicating that in neural cells GABAAR-mediated osmotic regulation affects the cellular volume thereby activating multiple intracellular signaling mechanisms important for cell proliferation, maturation, and survival. In addition, we will discuss evidence that the osmotic regulation exerted by GABA may contribute to brain water homeostasis in physiological and in pathological conditions causing brain edema, in which the GABAergic transmission is often altered.
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Affiliation(s)
- Tiziana Cesetti
- Department of Physiology and Pathophysiology, Interdisciplinary Center for Neurosciences, University of Heidelberg Heidelberg, Germany
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Le Bihan D. Diffusion, confusion and functional MRI. Neuroimage 2011; 62:1131-6. [PMID: 21985905 DOI: 10.1016/j.neuroimage.2011.09.058] [Citation(s) in RCA: 63] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2011] [Revised: 09/21/2011] [Accepted: 09/23/2011] [Indexed: 02/06/2023] Open
Abstract
Diffusion MRI has been introduced in 1985 and has had a very successful life on its own. While it has become a standard for imaging stroke and white matter disorders, the borders between diffusion MRI and the general field of fMRI have always remained fuzzy. First, diffusion MRI has been used to obtain images of brain function, based on the idea that diffusion MRI could also be made sensitive to blood flow, through the intravoxel incoherent motion (IVIM) concept. Second, the IVIM concept helped better understand the contribution from different vasculature components to the BOLD fMRI signal. Third, it has been shown recently that a genuine fMRI signal can be obtained with diffusion MRI. This "DfMRI" signal is notably different from the BOLD fMRI signal, especially for its much faster response to brain activation both at onset and offset, which points out to structural changes in the neural tissues, perhaps such as cell swelling, occurring in activated neural tissue. This short article reviews the major steps which have paved the way for this exciting development, underlying how technical progress with MRI equipment has each time been instrumental to expand the horizon of diffusion MRI toward the field of fMRI.
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Aso T, Urayama SI, Poupon C, Sawamoto N, Fukuyama H, Le Bihan D. An intrinsic diffusion response function for analyzing diffusion functional MRI time series. Neuroimage 2009; 47:1487-95. [PMID: 19450693 DOI: 10.1016/j.neuroimage.2009.05.027] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2008] [Revised: 04/19/2009] [Accepted: 05/11/2009] [Indexed: 11/26/2022] Open
Abstract
To disentangle the temporal profiles of the diffusion and BOLD components of diffusion-weighted functional MRI (DfMRI) during visual activation, we extracted the raw signal from an anatomically defined volume of interest encompassing the visual cortex of 16 subjects. Under the assumption of a linear, time invariant system we were able to define an intrinsic diffusion response function (DRF) from neural tissue, as a counterpart to the hemodynamic response function (HRF) commonly used in BOLD-fMRI. The shape of the DRF response was found to be very similar to the time courses of optical imaging transmittance signals, thought to originate from local geometric changes in brain tissue at the microscopic scale. The overall DfMRI signal response was modeled as the convolution of the stimulation paradigm time course with a DhRF, which is the sum of the DRF and a fractional HRF resulting from residual tissue T2-BOLD contrast. The contribution of the HRF to the DfMRI signal was found to be 26% at peak amplitude, but the DRF component which has a much steeper onset contributed solely at beginning of the response onset. The suitability of this model over the canonical HRF to process DfMRI data was then demonstrated on datasets acquired in 5 other subjects using a rapid event-related design. Some non-linearities in the responses were observed, mainly after the end of the stimulation.
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Affiliation(s)
- Toshihiko Aso
- Human Brain Research Center, Kyoto University Graduate School of Medicine, Kyoto, Japan.
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10
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Functional columns in the primate prefrontal cortex revealed by optical imaging in vitro. Neurosci Res 2008; 61:1-10. [DOI: 10.1016/j.neures.2008.01.003] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2007] [Revised: 01/06/2008] [Accepted: 01/08/2008] [Indexed: 11/24/2022]
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Fujiwara-Tsukamoto Y, Isomura Y, Imanishi M, Fukai T, Takada M. Distinct types of ionic modulation of GABA actions in pyramidal cells and interneurons during electrical induction of hippocampal seizure-like network activity. Eur J Neurosci 2007; 25:2713-25. [PMID: 17459104 DOI: 10.1111/j.1460-9568.2007.05543.x] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
It has recently been shown that electrical stimulation in normal extracellular fluid induces seizure-like afterdischarge activity that is always preceded by GABA-dependent slow depolarization. These afterdischarge responses are synchronous among mature hippocampal neurons and driven by excitatory GABAergic input. However, the differences in the mechanisms whereby the GABAergic signals in pyramidal cells and interneurons are transiently converted from hyperpolarizing to depolarizing (and even excitatory) have remained unclear. To clarify the network mechanisms underlying this rapid GABA conversion that induces afterdischarges, we examined the temporal changes in GABAergic responses in pyramidal cells and/or interneurons of the rat hippocampal CA1 area in vitro. The extents of slow depolarization and GABA conversion were much larger in the pyramidal cell group than in any group of interneurons. Besides GABA(A) receptor activation, neuronal excitation by ionotropic glutamate receptors enhanced GABA conversion in the pyramidal cells and consequent induction of afterdischarge. The slow depolarization was confirmed to consist of two distinct phases; an early phase that depended primarily on GABA(A)-mediated postsynaptic Cl- accumulation, and a late phase that depended on extracellular K+ accumulation, both of which were enhanced by glutamatergic neuron excitation. Moreover, extracellular K+ accumulation augmented each oscillatory response of the afterdischarge, probably by further Cl- accumulation through K+-coupled Cl- transporters. Our findings suggest that the GABA reversal potential may be elevated above their spike threshold predominantly in the pyramidal cells by biphasic Cl- intrusion during the slow depolarization in GABA- and glutamate-dependent fashion, leading to the initiation of seizure-like epileptiform activity.
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Affiliation(s)
- Yoko Fujiwara-Tsukamoto
- Department of System Neuroscience, Tokyo Metropolitan Institute for Neuroscience, 2-6 Musashidai, Fuchu, Tokyo 183-8526, Japan
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Abstract
Functional neuroimaging has emerged as an important approach to study the brain and the mind. Surprisingly, although they are based on radically different physical approaches both positron emission tomography (PET) and magnetic resonance imaging (MRI) make brain activation imaging possible through measurements involving water molecules. So far, PET and MRI functional imaging have relied on the principle that neuronal activation and blood flow are coupled through metabolism. However, a new paradigm has emerged to look at brain activity through the observation with MRI of the molecular diffusion of water. In contrast with the former approaches diffusion MRI has the potential to reveal changes in the intrinsic water physical properties during brain activation, which could be more intimately linked to the neuronal activation mechanisms and lead to an improved spatial and temporal resolution. However, this link has yet to be fully confirmed and understood. To shed light on the possible relationship between water and brain activation, this introductory paper reviews the most recent data on the physical properties of water and on the status of water in biological tissues, and evaluates their relevance to brain diffusion MRI. The biophysical mechanisms of brain activation are then reassessed to reveal their intimacy with the physical properties of water, which may come to be regarded as the 'molecule of the mind'.
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Affiliation(s)
- Denis Le Bihan
- NeuroSpin, Bâtiment 145, CEA Saclay, 91191 Gif-sur-Yvette, France.
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Goerke U, Möller HE. Transient signal changes in diffusion-weighted stimulated echoes during neuronal stimulation at 3T. J Magn Reson Imaging 2007; 25:947-56. [PMID: 17410563 DOI: 10.1002/jmri.20891] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
PURPOSE To develop a sensitive method for detecting minute transient signal changes that can arise due to variations in the extravascular apparent self-diffusion coefficient, D, during neuronal activation. MATERIALS AND METHODS A three-pulse sequence that reads out a moderately diffusion-weighted (DW) primary echo (PRE) and a heavily DW stimulated echo (STE) was employed to investigate whether small transient signal changes in extravascular D occur in response to a visual stimulus. Contributions to signal changes caused by subtle differences in the transient variations of the apparent transverse relaxation constant, T(2), between the PRE and STE were also quantified. RESULTS On z-maps obtained from the STE, more voxels showed significant stimulus-related signal changes compared to maps of the PRE. The average maximum signal change of the STE was larger than that of the PRE. The observed increase in the relative signal change was independent of the strength of the diffusion weighting. CONCLUSION The STE is more sensitive to neuronal activity than the PRE. The discrepancy between the two echoes does not arise from transient changes in D, but from subtle differences in stimulus-related variations of T(2) between the two echoes.
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Affiliation(s)
- Ute Goerke
- Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota, 2021 6th Street SE, Minneapolis, MN 55455, USA.
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Le Bihan D, Urayama SI, Aso T, Hanakawa T, Fukuyama H. Direct and fast detection of neuronal activation in the human brain with diffusion MRI. Proc Natl Acad Sci U S A 2006; 103:8263-8. [PMID: 16702549 PMCID: PMC1472461 DOI: 10.1073/pnas.0600644103] [Citation(s) in RCA: 202] [Impact Index Per Article: 11.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Using MRI, we found that a slowly diffusing water pool was expanding (1.7 +/- 0.3%) upon activation on the human visual cortex at the detriment of a faster diffusing pool. The time course of this water phase transition preceded the activation-triggered vascular response detected by usual functional MRI by several seconds. The observed changes in water diffusion likely reflect early biophysical events that take place in the activated cells, such as cell swelling and membrane expansion. Although the exact mechanisms remain to clarify, access to such an early and direct physiological marker of cortical activation with MRI will provide opportunities for functional neuroimaging of the human brain.
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Affiliation(s)
- Denis Le Bihan
- Human Brain Research Center, Kyoto University Graduate School of Medicine, Yoshida-Konoe-cho, Sakyo-ku, Kyoto 606-8501, Japan.
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Inoue H, Mori SI, Morishima S, Okada Y. Volume-sensitive chloride channels in mouse cortical neurons: characterization and role in volume regulation. Eur J Neurosci 2005; 21:1648-58. [PMID: 15845092 DOI: 10.1111/j.1460-9568.2005.04006.x] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Because persistent swelling causes cell damage and often results in cell death, volume regulation is an important physiological function in both neuronal and non-neuronal cells. Brain cell swelling has been observed not only in various pathological conditions but also during physiological synaptic transmissions. Volume-sensitive anion channels have been reported to play an important role in the regulatory volume decrease occurring after osmotic swelling in many cell types. In this study, using a two-photon laser scanning microscope and patch-clamp techniques, we found that mouse cortical neurons in primary culture exhibit regulatory volume decrease after transient swelling and activation of Cl- currents during exposure to a hypotonic solution. The regulatory volume decrease was inhibited by Cl- channel blockers or K+ channel blockers. Swelling-activated Cl- currents exhibited outward rectification, time-dependent inactivation at large positive potentials, a low-field anion permeability sequence, an intermediate unitary conductance and sensitivity to known blockers of volume-sensitive Cl- channels. Thus, it is concluded that the activity of the volume-sensitive outwardly rectifying Cl- channel plays a role in the control of cell volume in cortical neurons.
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Affiliation(s)
- Hana Inoue
- Department of Cell Physiology, National Institute for Physiological Sciences, Okazaki 444-8585, Japan
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Chavas J, Forero ME, Collin T, Llano I, Marty A. Osmotic Tension as a Possible Link between GABAA Receptor Activation and Intracellular Calcium Elevation. Neuron 2004; 44:701-13. [PMID: 15541317 DOI: 10.1016/j.neuron.2004.11.002] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2004] [Revised: 08/20/2004] [Accepted: 10/28/2004] [Indexed: 11/30/2022]
Abstract
Intracellular calcium concentration rises have been reported following activation of GABA(A) receptors in neonatal preparations and attributed to activation of voltage-dependent Ca(2+) channels. However, we show that, in cerebellar interneurons, GABA(A) agonists induce a somatodendritic Ca(2+) rise that persists at least until postnatal day 20 and is not mediated by depolarization-induced Ca(2+) entry. A local Ca(2+) elevation can likewise be elicited by repetitive stimulation of presynaptic GABAergic afferent fibers. We find that, following GABA(A) receptor activation, bicarbonate-induced Cl(-) entry leads to cell depolarization, Cl(-) accumulation, and osmotic tension. We propose that this tension induces the intracellular Ca(2+) rise as part of a regulatory volume decrease reaction. This mechanism introduces an unexpected link between activation of GABA(A) receptors and intracellular Ca(2+) elevation, which could contribute to activity-driven synaptic plasticity.
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Affiliation(s)
- Joël Chavas
- Laboratoire de Physiologie Cérébrale, CNRS UMR 8118, Université Paris 5, 45 rue des Saints Pères, 75006 Paris, France
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Isomura Y, Sugimoto M, Fujiwara-Tsukamoto Y, Yamamoto-Muraki S, Yamada J, Fukuda A. Synaptically activated Cl- accumulation responsible for depolarizing GABAergic responses in mature hippocampal neurons. J Neurophysiol 2004; 90:2752-6. [PMID: 14534278 DOI: 10.1152/jn.00142.2003] [Citation(s) in RCA: 90] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
It is known that GABA, a major inhibitory transmitter in the CNS, acts as an excitatory (or depolarizing) transmitter transiently after intense GABAA receptor activation in adult brains. The depolarizing effect is considered to be dependent on two GABAA receptor-permeable anions, chloride (Cl-) and bicarbonate (HCO3-). However, little is known about their spatial and temporal profiles during the GABAergic depolarization in postsynaptic neurons. In the present study, we show that the amplitude of synaptically induced depolarizing response was correlated with intracellular Cl- accumulation in the soma of mature hippocampal CA1 pyramidal cells, by using whole cell patch-clamp recording and Cl- imaging technique with a Cl- indicator 6-methoxy-N-ethylquinolinium iodide (MEQ). The synaptically activated Cl- accumulation was mediated dominantly through GABAA receptors. Basket cells, a subclass of fast-spiking interneurons innervating the somatic portion of the pyramidal cells, actually fired at high frequency during the Cl- accumulation accompanying the depolarizing responses. These results suggest synaptically activated GABAA-mediated Cl- accumulation may play a critical role in generation of an excitatory GABAergic response in the mature pyramidal cells receiving intense synaptic inputs. This may be the first demonstration of microscopic visualization of intracellular Cl- accumulation during synaptic activation.
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Affiliation(s)
- Y Isomura
- Department of System Neuroscience, Tokyo Metropolitan Institute for Neuroscience, Fuchu, Tokyo 183-8526, Japan.
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Abstract
Over most of their surface, neurons are surrounded by a narrow extracellular gap across which they make adhesive cell-cell contacts. Thus constrained, how do they regulate their geometry when osmotically perturbed? Specifically, are there any interesting consequences of local osmosis in such conditions? Using confocal imaging of shrinking neurons in culture, we observe water exiting into the cell-substratum gap. This water efflux generates a hydrostatic pressure that, at discrete (low adhesion) sites, causes the neuron's excess plasma membrane to invaginate, thus compensating for shrinkage with a pseudo-intracellular volume. To identify the minimal requirements of the process, a compartment/flux model was constructed. It comprises, essentially, a large liposome adhering in a labyrinthine fashion to a substratum. The model predicts that invaginations form at the cell-substratum interface under the influence of local osmosis, provided that adhesion across the gap is neither too tight nor too loose. Local osmosis in the central nervous system, in contrast to epithelia, is usually considered a mishap, not a physiological opportunity. We postulate, however, that local osmotic forces acting in conjunction with confined extracellular spaces could be harnessed in service of surface area, shape, and volume regulation when intense neural activity alters a neuron's osmotic balance.
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Affiliation(s)
- C E Morris
- Neuroscience, Ottawa Health Research Institute, Ottawa Hospital, Ottawa, Ontario, Canada.
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