用四甲基铵进行实时离子电渗疗法，以量化体细胞分数和脑细胞外空间的变性
1Department of Medicine,University of Virginia, 2Department of Cell Biology,SUNY Downstate Medical Center, 3Neural and Behavioral Science Graduate Program,SUNY Downstate Medical Center, 4Division of Neonatology,University of Virginia, 5Department of Neuroscience and Physiology,New York University School of Medicine
Summary
该方案描述了实时离子电渗疗法，一种测量活体大脑细胞外空间（ECS）物理参数的方法。释放到ECS中的惰性分子的扩散用于计算ECS体积分数和曲折度。它是研究脑ECS急性可逆变化的理想选择。
Abstract
该评论描述了执行实时离子电渗（RTI）方法的基本概念和方案，探索和定量活体大脑的细胞外空间（ECS）的金标准。 ECS围绕所有脑细胞并且包含间质液和细胞外基质。大脑活动所需的许多物质的运输，包括神经递质，激素和营养物质，都是通过ECS扩散发生的。这种空间的体积和几何形状的变化发生在正常的脑部过程，如睡眠和病理状况，如缺血。然而，大脑ECS的结构和调节，特别是在患病状态，仍然在很大程度上未被探索。 RTI方法测量活体大脑的两个物理参数：体积分数和曲折度。体积分数是ECS占据的组织体积的比例。柔软度是衡量物质在通过脑部扩散时遇到的相对阻碍与没有障碍物的介质相比。在RTI中，惰性分子从源微电极脉冲到脑ECS中。当分子从该源扩散时，使用离子选择性微电极定位大约100微米，离子的浓度随时间变化。从所得扩散曲线可以计算体积分数和曲折度。这种技术已被用于来自多个物种（包括人类）的脑切片和体内研究ECS的急性和慢性变化。与其他方法不同，RTI可以用于实时检测脑ECS的可逆和不可逆变化。
Introduction
细胞外空间（ECS）是所有脑细胞外部相互通路的网络，包含间质液和细胞外基质（ 图1a和图1b ）。脑细胞功能所需的许多物质（包括营养素，激素和神经递质）的分布通过ECS扩散发生。在该空间中的物理参数，包括体积，几何形状和细胞外基质，变化可以显着地通过ECS和局部离子浓度洗澡脑细胞，从而对脑细胞功能1,2产生深远的影响影响扩散。
实时离子电渗疗法（RTI）被用于确定一个大脑区域的两个结构特点：体积分数和弯曲度3，4，“xref”> 5。体积分数（ α ）是代表基础体积中ECS（ V ECS ）相对于总组织体积（ V 组织 ）占据的组织体积的比例;
与没有障碍物的介质相比，Tortuosity（ λ ）是通过脑区域扩散时物质遇到的相对障碍;
其中D * （cm 2 s -1 ）是物质在脑中的有效扩散系数， D （cm 2 s -1 ）是物质在游离介质如稀释琼脂糖凝胶中的自由扩散系数。
今天是R最常用的探针物质TI方法是小阳离子四甲基铵（TMA）。 TMA的分子量为74g / mol，在溶液中完全解离，并具有一个正电荷。这种离子的RTI研究已经证明α 
0.2和λ 1.6 1,2。这意味着ECS大约是总体积的20％，并且小的惰性分子的扩散在ECS中比在没有障碍物3的介质中大约慢2.5倍。然而， α和λ随着脑年龄，区域和状态以及病理状况而变化1 。这些参数的改变与大脑发育，衰老，睡眠，癫痫以及脑中许多其他基本过程和疾病有关1， 6 。虽然其他技术测量α和λ ，RTI可以实时测量生物组织的局部区域。因此，RTI在急性和可逆性挑战中已成为调查α和λ变化的不可缺少的工具。
该理论支撑器R最初由尼克尔森和Phillips验证，并且该技术已经被自那时起4,7广泛使用。使用RTI的实验开始于通过离子电渗法从源微电极释放到脉冲TMA的稀释琼脂糖凝胶。一旦弹出，离子从点源自由扩散，从潜在的无限数量的随机路径中选择（ 图1d ）。使用大约定位的离子选择性微电极（ISM）随时间测量离子的变化浓度100米远（ 图1c ）。 TMA浓度的变化被绘制并适合于允许计算D和离子电渗微电极（在方案中讨论的参数）的输送数的曲线。利用这些值，在感兴趣的脑区域中重复该过程以获得D *并计算α和λ 。离子电渗微电极的控制，TMA浓度曲线的数据收集，绘图和拟合以及实验参数的计算都通常由专门为此目的而设计的程序Wanda和Walter完成（软件及其手册是可根据要求自由提供）。
本评论的“议定书”部分描述了在啮齿动物脑切片中设计和执行RTI所需的基本程序。该技术也被用于无杆ENT模型，包括人脑切片和体内脑制剂1，4，6，8，9。 “代表结果”部分提供理想和非理想的结果，以突出数据解释的细微差别。最后，讨论部分简要介绍了故障排除技术，RTI的限制，用于研究ECS的替代技术以及RTI的未来应用。
图1：ECS扩散图。 （ a ）ECS图：在典型的脑部分显示ECS的大小和位置。黄色标记了灰色脑细胞过程之间的ECS。 ECS的体积大约是总组织体积的20％（ 即，体积分数= 0。2）生理条件下。 （ b ）ECS的放大图：重点介绍有助于曲折的物理参数，包括脑细胞几何（灰色）和细胞外基质（图示为多色糖胺聚糖和蛋白多糖的网格）。 （ c ）从点源扩散的3D图：证明惰性分子从离子电渗源到ISM的净运动。排除扩散阻挡层和细胞摄取，分子在所有方向向外扩散，产生球形浓度前沿。 ISM量化从离子电渗源释放的惰性分子的局部浓度。 （ d ）大脑ECS中扩散的计算机模拟：[左侧]蒙特卡罗模拟的设置;绿色球体代表脑细胞过程，红色十字表示点源。这个设置模拟图1a所示的脑组织。 [中图] 3和6个分子在通过大脑的细胞外空间扩散时进行随机运动，如图2所示。 [远]从点源释放的许多分子的随机游走。来自点源的所有分子的净运动是向外的， 如图1c所示 。累积随机散点概述了单元格之间的空格（ 即 ECS;参见参考文献5进行进一步说明）。 请点击此处查看此图的较大版本。
Protocol
用于获取组织样本的所有动物手术均获得SUNY Downstate医疗中心动物伦理委员会的批准。 1.准备解决方案和设备为ISM的参考筒准备150 mM NaCl回填溶液。将其储存在连接到0.22μm过滤器上的10mL注射器中（以去除细菌或颗粒）。 为微电极制备150 mM TMA氯化物（TMA-Cl）回填溶液。将其储存在连接到0.22μm过滤器上的10mL注射器中。从5 M制造商库存解决方案准备TMA-C1溶液（在本…
Representative Results
RTI技术的实用性在设计用于测量α和低渗摩洛尔挑战期间变化的实验中得到证明（ 图8和图9 ）。先前已经表明，降低了ECS的渗透压通过洗涤上低渗ACSF将产生α的降低，并增加了λ13。 在该实验中，在两个对照条件下和在低渗ACSF的洗涤期间对大?…
Discussion
图10：展示常见技术问题的非理想数据。 （ a ）离子电渗微电极的常见技术问题图：功能性离子电渗微电极的TMA正常释放与三个来源的技术问题的比较。 [高放大倍数，a1]理想的离子电渗源中的电流通过TMA释放和氯化物摄取而均匀地进行。 [高倍率，A2]一种离子电渗微电极具有低N t个释放更少…
Disclosures
The authors have nothing to disclose.
Acknowledgements
这项工作得到NIH NINDS授予R01 NS047557的支持。
Materials
| A/D and D/A converter | National Instruments Corporation | NI USB-6221 DAQ | The NI USB-6221 is still sold as a 'Legacy' device by NI. They recommend using NI USB-6341 X Series DAQs for new installations, however we have not tested the newer units. We describe the use of the NI USB-6221 with MATLAB and Windows 7 (32-bit). Alternatives: the much older PCI-MIO-16E-4 A/D converter (Used under Windows XP or older OS only) with BNC-2090 BNC connector panel and SH68-68-EP cable. As noted in the Wanda Manual, an experimental MATLAB program to use Axon Binary Files is available. |
| agarose | Lonza | NuSieve GTG Agarose #50081 | to prepare dilute agarose gel for RTI measurements |
| amplifier for ISM | Dagan | Model IX2-700 Dual Intracellular Preamplifier | ion and reference voltage amplifier with N=0.1 (for reference barrel) and N=0.001 (for ion barrel) headstages |
| biological compound miscroscope (with 4x and 10x objective) | for chipping the microelectrode tips and inspecting microelectrodes; various suppliers, e.g. AmScope | ||
| borosilicate theta capillary glass tubing | Harvard Apparatus | Warner Instruments model TG200-4; order #64-0811 | double-barreled glass tubing for ion-selective microelectrodes and iontophoretic microelectrodes; O.D. 2.0 mm, I.D. 1.4 mm, septum 0.2 mm, length 10 cm |
| brush | Winsor & Newton | University Series 233, size 0 | round shoft handle brush, available from Amazon |
| bunsen burner | Fisher | ||
| camera for visualizing micropipettes | Olympus | OLY-150 | requires monitor, IR filter on substage illuminator is optional |
| chart recorder | to record continuously voltages on ion-selective microelectrode during calibration in tetramethylammonium standards and during RTI experiment; e.g. Kipp & Zonen type BD112 dual-cannel chart recorded, available refurbished | ||
| chlorotrimethylsilane, puriss., > 99% | Sigma-Aldrich | catalog # 92360 | for silanization; CAUTION: flammable, acute toxicity (oral, dermal, inhalation), skin corrosion, eye damage, reacts violently with water, see Sigma-Aldrich Safety Information for full description |
| Commercial Software | The MathWorks | MATLAB, Data acquisition toolbox | for data acquisition and analysis using Wanda and Walter programs. Note that an academic license is available. |
| eye protective goggles | Fisher | ||
| fixed-stage compound microscope | Olympus | BX51WI | can use other compound microscopes with fixed stages |
| forceps | Fine Science Tools | #11251-10 | to chip glass capillary; Dumond #5, preferably used and no longer needed for fine work |
| fume hood | for silanization and filling the tip of ion-selective barrel with liquid ion exchanger; various supliers, e.g. Captair with approriate filter sold by Erlab | ||
| glass microscope slide | Fisher | #12-550A | to chip microelectrode tips |
| heater/stirrer | Fisher | Corning PC-420D | to prepare dilute agarose gel and stir solutions |
| iontophoretic unit | Dagan | ION-100 and PS-100 | ION-100 is a single channel iontophoresis unit +/- 130 V compliance; PS-100 is an external power supply; alternatives: e.g. Axoprobe-1A made by Axon Instruments (now Molecular Devices), out of production, check for availability of refurbished units (eBay and other sites) |
| liquid ion exchanger (LIX) for tetramethylammonium | World Precision Instruments | IE190 Potassium Ion Exchanger | Note: this is equivalent to the original Corning potassium exchanger 477317 based on tetraphenlyborate – do not confuse with neutral carrier potassium exchanger originating from the laboartory of Dr. Simon, ETH, Zurich, which does not sense tetramethylammonium, and is sold by Fluka. You can also make liquid ion exchanger for tetramethylammonium yourself: 3% by weight potassium tetrakis = (p-chlorophenyl) borate dissolved in 2,3-dimethylnitrobenzene. Buy chemicals from Fluka (now part of Sigma). See Oehme and Simon (1976) Anal. Chim. Acta 86: 21-25; CAUTION: The toxicological properties of this liquid ion exchanger have not been fully determined. Ingestion or contact with the human body may be harmful. Exercise due care! Liquid ion exchangers should be stored in a cool place out of direct sunlight. |
| microelectrode holder | WPI | M3301EH | to hold ion-selective microeletrode prefabricate for silanization and filling the tip of ion-selective barrel with liquid ion exchanger; WPI sells two versions of this holder, clear M3301EH and black M3301EH. In our experience, the clear M3301EH appears to be sturdier then the black M3301EH. |
| micromanipulator | Narishige | MM-3 | to position ion-selective microelectrode prefabricate during silanization and filling the tip of ion-selective barrel with liquid ion exchanger; can be substituted with any three-axis micromanipulator in good working condition |
| micropipette puller | Sutter Instruments | Model P-97 | to pull double-barreled glass tubing; other pullers can be used as long as they can accommodate large diameter double-barreled glass tubing |
| microprobe thermometer | Physiotemp | Model BAT-12R | fine probe of this thermometer is placed close to recording site |
| needle | BD | Syringes and Needles # 305122 (25 gauge) | for silanization; BD PrecisionGlide needles 25 G x 5/8 in (0.5mm x 16mm) |
| objective 5x dry | Olympus | MPlan N | |
| objective 10x water immersion | Olympus | UMPlan FL N | 10x objective is water immersion, numerical aperture is 0.3, working distance is 3.3 mm |
| plastic containers (with lids) | Fisher | #14-375-148 | to store tetramethylammonium standard solutions and microelectrodes |
| platform and x-y translation stage for fixed-stage microscope | EXFO | Gibraltar Burleigh | platform holds slice chamber, micromanipulators and accesorries, x-y translational stage moves microscope without compromising recording stability |
| porous minicup | for RTI measurements in a dilute agarose gel; homemade | ||
| reusable adhesive | Bostik | Blu-Tack | for securing microelectrodes to holding vessel and other uses; various suppliers, available from Amazon |
| robotic micromanipulator with precise x,y,z positioning | Sutter Instruments | MP-285 | two mircomanipulators are needed to hold separately ion-selective microelectrode and iontophoretic microelectrode. Also possible to glue micropipettes in a spaced array (see text). |
| signal conditioning unit with low-pass filter | Axon Instruments | CyberAmp 320 or 380 | no longer available from the manufacturer but may be available from E-Bay; alternatives: e.g. FLA-01 Filter/Amplifier from Cygnus Technology. This is a single channel instrument with a minimum cutoff at 10 Hz using a multipole Bessel filter but the company may be willing to modify it for a lower cutoff frequency (2 Hz) if needed. |
| silver wire | A-M Systems | #7830 | diameter 0.015", bare (no coating) |
| slice chamber | Harvard Apparatus | Warner Model RC-27L | this is submersion slice chamber; do not use interface slice chamber |
| stereomicroscope | for silanization and filling the tip of ion-selective barrel with liquid ion exchanger; horizontally mounted; various suppliers | ||
| syringe, 10 mL | BD | Syringes and Needles #309604 | to backfill microelectrodes and for silanization; BD Luer-Lok tip |
| syringe filter 0.22µm pore | Whatman | #6780-1302 | to filter backfill solutions; available from Fisher |
| syringe needle, 28 gauge, 97mm | World Precision Instruments | MicroFil MF28G-5 | to backfill microelectrodes |
| Teflon (=PTFE) tubing | Component Supply | STT-28 PTFE tube light wall (28 gauge) | for silanization of ion-selective barrel; fits on BD PrecisionGlide needles 25 G x 5/8 in. Note: Teflon is essential, PVC tubing would melt by hot wax. |
| temperature control system | Harvard Apparatus | Warner Models TC-344B and SH-27A | TC-344B is a dual automatic temperature controller, SH-27A is an in-line heater; controller and heater work with Warner slice chambers |
| tetramethyammonium (TMA) chloride | Sigma-Aldrich | T-3411 | 5 M solution; CAUTION: acute toxicity (oral, dermal, inhalation), carcinogenicity, hazardous to the aquatic environment, see Sigma-Aldrich Safety Information for full description |
| vibrating blade microtome | Leica | VT1000S | to cut brain slices |
| xylenes | Fisher | X5-1 | for silanization; CAUTION: flammable, acute toxicity (oral, dermal, inhalation), skin corrosion, eye damage, carcinogenicity, see Fisher Safety Information for full description |
References
- Sykova, E., Nicholson, C. Diffusion in brain extracellular space. Physiol Rev. 88 (4), 1277-1340 (2008).
- Nicholson, C. Diffusion and related transport mechanisms in brain tissue. Rep Prog Phys. 64 (7), 815-884 (2001).
- Nicholson, C. Ion-selective microelectrodes and diffusion measurements as tools to explore the brain cell microenvironment. J Neurosci Methods. 48 (3), 199-213 (1993).
- Nicholson, C., Phillips, J. M. Ion diffusion modified by tortuosity and volume fraction in the extracellular microenvironment of the rat cerebellum. J Physiol. 321, 225-257 (1981).
- Nicholson, C., Sykova, E. Extracellular space structure revealed by diffusion analysis. Trends Neurosci. 21 (5), 207-215 (1998).
- Xie, L. L., et al. Sleep drives metabolite clearance from the adult brain. Science. 342 (6156), 373-377 (2013).
- Hrabetova, S., Nicholson, C., Michael, A. C., Borland, L. M. Biophysical properties of brain extracellular space explored with ion-selective microelectrodes, integrative optical imaging and related techniques. Electrochemical Methods for Neuroscience Neuroscience. , 167-204 (2007).
- Rice, M. E., Okada, Y. C., Nicholson, C. Anisotropic and heterogeneous diffusion in the turtle cerebellum: implications for volume transmission. J Neurophysiol. 70 (5), 2035-2044 (1993).
- Vargova, L., et al. Diffusion parameters of the extracellular space in human gliomas. Glia. 42 (1), 77-88 (2003).
- Haack, N., Durry, S., Kafitz, K. W., Chesler, M., Rose, C. Double-barreled and concentric microelectrodes for measurement of extracellular ion signals in brain tissue. J Vis Exp. (103), (2015).
- Xiao, F., Hrabetova, S. Enlarged extracellular space of aquaporin-4-deficient mice does not enhance diffusion of Alexa Fluor 488 or dextran polymers. Neuroscience. 161 (1), 39-45 (2009).
- Sherpa, A. D., Pvan de Nes, ., Xiao, F., Weedon, J., Hrabetova, S. Gliotoxin-induced swelling of astrocytes hinders diffusion in brain extracellular space via formation of dead-space microdomains. Glia. 62 (7), 1053-1065 (2014).
- Kume-Kick, J., et al. Independence of extracellular tortuosity and volume fraction during osmotic challenge in rat neocortex. J Physiol. 542 (Pt 2), 515-527 (2002).
- Saghyan, A., Lewis, D. P., Hrabe, J., Hrabetova, S. Extracellular diffusion in laminar brain structures exemplified by hippocampus. J Neurosci Methods. 205 (1), 110-118 (2012).
- Fedirko, N., Svichar, N., Chesler, M. Fabrication and use of high-speed, concentric H+- and Ca2+-selective microelectrodes suitable for in vitro extracellular recording. J Neurophys. 96 (2), 919-924 (2006).
- Nicholson, C. Diffusion from an injected volume of a substance in brain tissue with arbitrary volume fraction and tortuosity. Brain Res. 333 (2), 325-329 (1985).
- Nicholson, C., Tao, L. Hindered diffusion of high molecular weight compounds in brain extracellular microenvironment measured with integrative optical imaging. Biophys J. 65 (6), 2277-2290 (1993).
- Thorne, R. G., Nicholson, C. In vivo diffusion analysis with quantum dots and dextrans predicts the width of brain extracellular space. Proc Natl Acad Sci U S A. 103 (14), 5567-5572 (2006).
- Wolak, D. J., Thorne, R. G. Diffusion of macromolecules in the brain: implications for drug delivery. Mol Pharm. 10 (5), 1492-1504 (2013).
Cite This Article
Odackal, J., Colbourn, R., Odackal, N. J., Tao, L., Nicholson, C., Hrabetova, S. Real-time Iontophoresis with Tetramethylammonium to Quantify Volume Fraction and Tortuosity of Brain Extracellular Space. J. Vis. Exp. (125), e55755, doi:10.3791/55755 (2017).