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Neuroscience

A Micro-CT-based Method for Characterizing Lesions and Locating Electrodes in Small Animal Brains

Published: November 8th, 2018

DOI:

10.3791/58585

1Department of Molecular and Cellular Biology, Harvard University, 2Center for Brain Science, Harvard University, 3Department of Organismic and Evolutionary Biology, Harvard University, 4Institute of Science and Technology Austria

This article describes a straightforward method to prepare small animal brains for micro-CT imaging, in which lesions can be quantified and electrodes located with high precision in the context of the whole brain.

Lesion and electrode location verification are traditionally done via histological examination of stained brain slices, a time-consuming procedure that requires manual estimation. Here, we describe a simple, straightforward method for quantifying lesions and locating electrodes in the brain that is less laborious and yields more detailed results. Whole brains are stained with osmium tetroxide, embedded in resin, and imaged with a micro-CT scanner. The scans result in 3D digital volumes of the brains with resolutions and virtual section thicknesses dependent on the sample size (12–15 and 5–6 µm per voxel for rat and zebra finch brains, respectively). Surface and deep lesions can be characterized, and single tetrodes, tetrode arrays, electrolytic lesions, and silicon probes can also be localized. Free and proprietary software allows experimenters to examine the sample volume from any plane and segment the volume manually or automatically. Because this method generates whole brain volume, lesions and electrodes can be quantified to a much higher degree than in current methods, which will help standardize comparisons within and across studies.

Neuroscientists have relied on lesions for a long time in order to understand the relationship between function and location in the brain. For example, our understanding of the hippocampus as being indispensable for learning and memory and of the prefrontal cortex as being key for impulse control were both products of serendipitous lesions in humans1,2. The use of animal models, however, has allowed neuroscientists to harness the power of lesions by going beyond serendipity, and the function of countless brain areas has been elucidated through systematic studies of structure-function relationships through lesi....

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All care and experimental manipulation of animals were reviewed and approved by the Harvard Institutional Animal Care and Use Committee. The perfusion described here is specific for rats, but the procedure is applicable to any animals with smaller or similarly sized brains.

1. Perfusion

  1. Prepare 1x phosphate-buffered saline (PBS). For a rat (age: 0.5–1.5 years old, weight: 250–600 g), 800–1,000 mL should be sufficient. Use 400 mL to perfuse the animal and an additi.......

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Traditionally, brains are sectioned and stained in order to quantify lesions and locate electrodes, but this method is error-prone, labor-intensive, and typically requires estimation of the results. By preparing whole brains for micro-CT imaging, the probability of damaging the samples is greatly reduced, features of interest may be analyzed in the context of the entire brain, and the method lends itself to parallel processing of many samples, considerably speeding up sample preparation.<.......

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The following are critical steps to the protocol: first, the use of a combination of PFA and GA to perfuse the animal and subsequently post-fix the brain was paramount to achieving consistent full osmium penetration of the tissue. Although we did not test this explicitly, a plausible explanation is that PFA fixation is reversible15, whereas GA fixation is not reversible16,17. Because a two-week incubation in osmium tetroxide is required fo.......

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The authors thank Greg Lin and Arthur McClelland for their expertise with the micro-CT machine, David Richmond and Hunter Elliott at the Image and Data Analysis Core (IDAC) at Harvard Medical School for their image processing advice, and William Liberti at Boston University for graciously providing a zebra finch brain. This work was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award no. 1541959. CNS is a part of Harvard University. This work was supported by the Richard and Susan Smith Family Foundation and ....

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Name Company Catalog Number Comments
Paraformaldehyde (PFA) Electron Microscopy Sciences (EMS) 15710 2% (w/v/) in 1X PBS
Glutaraldehyde (GA) EMS 16220 2.5% (w/v) GA in 1X PBS
OsO4 EMS 19190 Work in fume hood
Ethanol Decon Labs Koptec 140, 190, 200 proof
Acetone EMS 10015 Glass-distilled
Durcupan ACM resin Sigma-Aldrich 44610 A, B, C and D components, resin for embedding
Disposable molds Ted Pella 27114 Suggested
milliQ water (ultrapure water) Millipore Sigma QGARD00R1 (or related purifier) Suggested
Parafilm (paraffin film) Millipore Sigma P7793 Suggested paraffin film
Micro-CT scanner Nikon Metrology Ltd., Tring, UK X-Tek HMS ST 225 Used by authors
Software for visualizing and analyzing micro-CT scans:
Volume Graphics VG Studio Max Used by authors
FEI / Thermo Scientific Avizo Used by authors
FEI / Thermo Scientific Amira Similar to Avizo
Mark Sutton & Russell Garwood Spiers Free, http://spiers-software.org/
Pixmeo Sarl Osirix Lite Free, https://www.osirix-viewer.com/
Open Source FIJI Free, https://fiji.sc/
Adobe Photoshop Good for analyzing one slice at a time

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