Paw-Dragging: a Novel, Sensitive Analysis of the Mouse Cylinder Test

Summary

Classical forelimb asymmetry analysis of the cylinder test is routinely used to assess behavioural deficits in rats following brain injury or stroke; however, it fails to detect consistent deficits in mice. This study demonstrates that quantifying paw-dragging behaviour is a more sensitive analysis of brain injury in mice.

Abstract

The cylinder test is routinely used to predict focal ischemic damage to the forelimb motor cortex in rodents. When placed in the cylinder, rodents explore by rearing and touching the walls of the cylinder with their forelimb paws for postural support. Following ischemic injury to the forelimb sensorimotor cortex, rats rely more heavily on their unaffected forelimb paw for postural support resulting in fewer touches with their affected paw which is termed forelimb asymmetry. In contrast, focal ischemic damage in the mouse brain fails to result in comparable consistent deficits in forelimb asymmetry. While forelimb asymmetry deficits are infrequently observed, mice do demonstrate a novel behaviour post stroke termed “paw-dragging”. Paw-dragging is the tendency for a mouse to drag its affected paw along the cylinder wall rather than directly push off from the wall when dismounting from a rear to a four-legged stance. We have previously demonstrated that paw-dragging behaviour is highly sensitive to small cortical ischemic injuries to the forelimb motor cortex. Here we provide a detailed protocol for paw-dragging analysis. We define what a paw-drag is and demonstrate how to quantify paw-dragging behaviour. The cylinder test is a simple and inexpensive test to administer and does not require pre-training or food deprivation strategies. In using paw-dragging analysis with the cylinder test, it fills a niche for predicting cortical ischemic injuries such as photothrombosis and Endothelin-1 (ET-1)-induced ischemia – two models that are ever-increasing in popularity and produce smaller focal injuries than middle cerebral artery occlusion. Finally, measuring paw-dragging behaviour in the cylinder test will allow studies of functional recovery after cortical injury using a wide cohort of transgenic mouse strains where previous forelimb asymmetry analysis has failed to detect consistent deficits.

Introduction

The goal of neural regeneration strategies is to demonstrate both tissue repair and functional recovery. Functional recovery is typically evaluated with behavioural tests that measure functional deficits, in this case involving motor skills that are associated with damage to the specific brain regions. Traumatic brain injury or ischemic damage to the sensorimotor forelimb area of the cortex can be demonstrated by a number of behavioural tests. One such test, the cylinder test is used extensively in rats to assess functional deficits in forelimb activity¹. The test has a low set-up cost requiring only a cylinder, camera and table with a transparent top. It is easy to administer as it is based on the natural exploratory behaviour of rodents, so pre-training and food deprivation or rewards are not required. Despite these numerous advantages, the cylinder test is under-utilized to assess forelimb deficits in mice following focal injuries to the forelimb sensorimotor cortex, which we attribute to the analysis of mouse behaviour in the cylinder test. Forelimb asymmetry is the classical measure of analysis for the cylinder test. When placed in the cylinder, rodents naturally explore the walls of the cylinder by rearing onto their hind limbs and touching the cylinder walls with their forelimb paws for postural balance. The number of paw touches with the wall with each forelimb is easily quantified by filming rodents during this exploration of the cylinder. Forelimb asymmetry occurs when the affected forelimb paw makes fewer touches with the wall than the unaffected forelimb paw and is indicative of damage to the contralateral sensorimotor cortex. In rats, intra-cortical injections of the vasoconstrictive agent, Endothelin (ET-1), into the forelimb sensorimotor cortex causes a focal ischemic lesion which results in behavioural deficits in the contralateral forelimb. Deficits in contralateral forelimb use are readily detected as changes in forelimb asymmetry in the cylinder test in rats^1-3. In contrast to rats however, changes in forelimb asymmetry are variable and less consistent in mice following comparable ET-1 injections^4-6. Here we demonstrate a novel analysis of forelimb behaviour in the cylinder test – analysis of paw-dragging behaviour. We have previously shown that paw-dragging analysis is a more sensitive measure of damage to the forelimb sensorimotor cortex in mice than the classical forelimb asymmetry analysis and therefore is applicable to a variety of focal cortical injury models.

Examination of how the forepaw contacts the cylinder wall following ischemic damage to the forelimb sensorimotor cortex revealed a novel behaviour in mice - paw-dragging⁴. A paw-drag occurs when a mouse stands on its rear legs to explore the cylinder wall then drags its affected (contra-lesional) paw along the cylinder wall towards its midline or down the wall while its unaffected forepaw provides postural support against the wall. Paw-drags rarely occur in uninjured mice therefore the appearance of a paw-drag is a positive indicator of injury to the forelimb sensorimotor cortex⁴. We have previously quantified paw-dragging behaviour in mice following ET-1 ischemic damage to the forelimb sensorimotor cortex and have shown sustained paw-dragging behaviour in mice up to two weeks post-stroke⁴. Here we show that paw-dragging behaviour is sustained up to four weeks post-stroke. Analysis of paw-dragging behaviour provides a novel and sensitive tool for assessing focal ischemic damage to the forelimb sensorimotor cortex in mice. Its inexpensive set-up, ease of administration and scoring make this a simple, yet useful tool to rapidly assess forelimb behavioural deficits in mice.

Protocol

Ethics statement: All experiments were approved by Memorial University of Newfoundland's Animal Care Ethics Committee according to the guidelines of the Canadian Council on Animal Care.

1. Mice

1. Use adult mice. In this study, adult male FVBN mice (n=10) between 2-4 months were used. House mice on a 12:12 hr reverse light-dark cycle and provide standard rodent chow and water ad libitum.

2. Materials Required for the Cylinder Test

1. Obtain a table with a transparent top to film the cylinder test. The dimensions of the table are irrelevant, the top must be plexiglass or glass, and there must be enough room to position a mirror below the table. This allows for the mouse to be videotaped from below. Use a mirror below the table to reflect the image through the cylinder. As an alternative, use an upside-down camera if available. The dimensions of the table used in this protocol are 54 x 56 x 66.5cm (w x l x h) with a 51 x 51cm top (w x l).
2. Obtain a mirror. The dimensions of the mirror used in this protocol are 34 x 58cm (w x l).
3. Obtain a transparent/plexiglass cylinder for the mouse to perform in. The dimensions of the cylinder used in this protocol are 17.5cm high, 8.8cm I.D, 9.5cm O.D with a wall thickness of 0.35cm. A taller cylinder may be required for more active mouse strains.
4. Place the cylinder on the tabletop and film the reflection in the mirror below.
5. A videocamera and tripod are required. Record videos at approximately 650Kb/s, which is approximately 190Mb per 5 min of cylinder video. Ensure that the camera has a zoom functionality to ensure that the cylinder encompasses the entire field of view.
   NOTE: The videocamera used in this protocol is a Sony DCR-SR42, 40x optical zoom, 2,000x digital zoom, 680kpix which uses standard definition, NTSC interlaced video.)
6. Obtain software for analysis – a media player with video support and playback speed modulation. The media player used in this protocol is the VLC Media Player v2.1.2.
7. Obtain a computer with an operating system capable of running the media player and a monitor.
8. Electronic storage for videos is required. Download videos to an external hard drive or copy it onto DVDs for long-term storage.
   NOTE: At 190Mb per video, 84 sessions will fit on a 16 GB SD card and 168 sessions will fit on a 32GB SD card. Due to the relative inexpensiveness of SD media, and the uncertainty in how much time some mice require to complete 20 rears, a 32 GB card is recommended. In the current study, the videos were copied from the camera to a 2TB external hard drive and then recopied onto DVDs as backup.

3. Experimental Setup of the Cylinder Test

1. Fasten mirror below the table at a 45 degree angle to the tabletop. Do this using two support brackets attached to the table legs to support the top and bottom of the mirror, respectively. Bracket locations are noted in a side-view of the table (Figure 1A) and a face-on view of the table (Figure 1B).
2. Place cylinder on the center of the table. Draw four equidistant lines where the cylinder sits on the table with a black marker (Sharpie) so that the cylinder can be lifted and returned to the same position (Figure 2). Drawing them on the underside of the transparent tabletop allows the tabletop to be cleaned between animal tests without dissolving the marker ink.
3. Attach the camera and tripod. Aim the camera at the mirror so that the picture is looking directly through the barrel of the cylinder. Ensure that the full inner wall of the cylinder is visible and unobstructed by the base (Figure 3). See a side-on view of the setup, including the relative angle of the camera and mirror setup, from above (Figure 4A) and from the level of the table, including a mouse rearing (Figure 4B).
4. Prepare cue cards to identify each mouse prior to filming. Ensure that the cards typically include an identification number for each mouse, the time-point of the test (e.g., 3 days after treatment) and the date of the filming session. Do not include the treatment group on the cue card to ensure the experimenter is blinded.
5. Film in standard indoor lighting conditions as this level of light is required to clearly see the mouse movements around the cylinder.
   NOTE: If available filming in the dark with a red light camera may suffice, however one would first need to test whether paw touches and drags are clearly visible for quantification.

4. Execution

1. Start filming. Display the appropriate mouse cue card in front of camera lens.
2. Lower the mouse into the cylinder from the open top immediately after filming the cue card.
3. Begin filming the mouse. Minimize noise during this time, as mice may lose interest in exploring if startled.
4. Observe the mice rear to explore the cylinder. Capture the video until the mouse performs a minimum of twenty rears.
   NOTE: A rear occurs when both forepaws lose contact with the floor and the mouse stands on its hind legs.
5. Wipe tabletop and cylinder with an appropriate cleaning solution between each mouse to sanitize and remove mouse scents.
6. Freezing is when mice stop exploring and remain quietly sitting on all fours for approximately 5 min. If mice freeze before twenty rears occur, it may be necessary to remove them from the cylinder for 10-20 min before resuming the test. If mice do not perform twenty rears, they are removed from the study.
   NOTE: In our experience, mice have never needed to be excluded due to failure to explore the cylinder.

5. Evaluation of the Cylinder Test using Paw-Dragging Analysis

1. Play back the video at a rate of between 0.25x and 0.67x regular speed depending on how quickly the mouse explores the cylinder. Use a media player that offers slower playback speeds.
2. Quantify the total number of paw touches. Paw touches occur when the mouse rears (Figure 5A), touches the side of the cylinder (Figure 5B), subsequently dismounts with both paws simultaneously (Figure 5C) and lands (Figure 5D). The paw may or may not contact the cylinder wall with a full palm, but some contact with the cylinder wall must occur. Assess paw touches by counting the number of times the mouse makes contact (no matter how brief) with the cylinder wall with either or both forepaws while standing on its hind limbs during a rear.
   Note that contact with the cylinder wall is only counted as a “paw touch” or “paw-drag” if the mouse is in a rear position – standing on its hind limbs with both forepaws off of the tabletop. If the mouse remains in a 3-point stance – both hind limbs and one forepaw on the tabletop and proceeds to touch the wall with the free paw – this is not counted as a paw touch. Mice may rear and touch the cylinder wall with a single forepaw and this is counted as a paw touch. Note: a mouse may also move its body around the cylinder during a rear, making more than two contacts. These contacts are tallied – one for each left forepaw touch and one for each right forepaw touch.
3. Quantify the number of paw-drags. Paw-dragging behaviour is distinct from normal paw touches.
   1. If the paw contacts the cylinder wall with a full open palm (Figure 6B), it will slowly fall away from the wall, often with a slight tremor. The movement begins with the digits dragging against the cylinder wall either in a medial or downward direction, (Figure 6C) before falling away completely (Figure 6D). The mouse will then dismount with its unaffected paw (Figure 6E) before landing on all fours (Figure 6F). This is considered a paw-drag and should be counted in a tally.
   2. If the paw does not contact the cylinder wall with a fully open palm, it will graze the cylinder wall with its digits before falling away from the cylinder wall. Similarly, a mouse may drag its paw against the cylinder wall but not release it entirely before dismounting. These are both considered paw-drags as well as touches and should be counted as both in a tally.
   3. The paw may also drag along the cylinder wall while a mouse explores the cylinder. In this case, the paw will follow the twisting of the mouse’s torso as it explores left or right of its original position (Figure 7A-D) before dismounting (Figure 7E). This is not considered a paw-drag, as it depends on the mouse randomly choosing a direction to explore and does not depend on which cortical hemisphere was damaged.
4. Paw-drags are expressed as a percentage of paw-drags per total number of paw touches during a session. Express the number of paw-drags as a percentage of total paw contacts for each forelimb separately.
5. Touches resulting in a paw-drag count as a paw-drag and a touch simultaneously. Thus, if a mouse drags its paw each time its paw contacts the cylinder wall, the paw-dragging percentage is expressed as 100%.

6. Additional Experimental Design Suggestions

1. To minimize extraneous variables:
   1. Test the mice at the same time on each testing day. Test the mice during their wake cycle. Keeping mice on a 12 hr reverse light cycle facilitates performance.
   2. Mice may be reluctant to explore the cylinder if stressed either by noise or a novel environment. Testing mice in their animal holding room or a room they have been familiarized with reduces stress. This can occur if the room is noisy, if the mouse has been jostled prior to entering the cylinder or due to habituation.
      NOTE: Testing should be performed once before experimental manipulation to serve as a baseline reading. After manipulation, testing days are at the experimenter’s discretion, though it is advised to avoid an excessive number of exposures to the cylinder over a short period of time.
   3. Mice may become reluctant to rear after 6-7 exposures to the cylinder. For the current study, mice were tested in the cylinder for a total of seven times, prior to ischemia and on days 1, 3, 7, 14, 21 and 28 post-surgery.
      NOTE: In this study we used the FVBN mouse strain. We have previously tested C57Bl/6 mice in the cylinder and observed paw-dragging behaviour following an ET-1 ischemic injury (data not shown). C57BL/6 mice were more active than FVBN mice when rearing and frequently jumped onto the rim of the cylinder before climbing out. Taller cylinders should be used if mice attempt to escape by jumping.

7. Endothelin-1 Surgery and Infarct Volume Measurements

1. Perform Endothelin-1 surgery and infarct volume measurements according to previously published protocols⁴ . To target the anterior forelimb motor cortex, each mouse should receive three ET-1 injections at the following coordinates: (i) +0.7 anteriorposterior (AP)/1.5 medial-lateral (ml)/-1.2 dorsal-ventral (DV), (ii) +0.4 AP/1.25 ml/- 1.2 DV and (iii) +0.1 AP/1.75 ml/-1.2 DV⁴ .

8. Statistical Analysis

1. A two-way repeated measures analysis of variance (ANOVA) is recommended to analyze the percent of paw-dragging for the affected and unaffected paws across different time points.

Results

We have previously demonstrated that paw-dragging behaviour appears following a focal ischemic injury to the forelimb sensorimotor cortex and is a positive indicator of damage⁴. Intra-cortical injections of ET-1 into the forelimb sensorimotor cortex were used to induce an ischemic lesion (Figure 8A,B). This study examined whether paw-dragging behaviour extended for longer than 14 days post-injury for its potential use to assess functional recovery. Mice were tested in the cylinder test on the ...

Discussion

The key points to establish when quantifying paw-dragging behaviour in the cylinder test are the following: i) quantify the number of paw-drags versus total paw touches for each paw before brain injury to establish a baseline; ii) quantify the number of paw-drags versus total paw touches for each paw following the ischemic injury; and iii) discriminate between a paw-drag and the lateral motion of the paw along the cylinder wall during lateral rotation of the mouse’s torso.

Paw-dragging i...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Plexi-glass cylinder			17.5 cm high, 9.5 cm outer diameter, 8.8 cm inner diameter, wall thickness 0.35 cm (or 3.5 mm)
viewing table			54x56x66.5 cm (width x length x height), top of table is a 51x51 cm sheet of plexiglass.
mirror			34x58 cm mirror
video camera	Sony	DCR-SR42	Video camera with onboard storage, SD functionality, 40x optical zoom
computer	Dell	Optiplex 760	Processor: Intel, 3.0 GHz, Memory 4.00GB (RAM)
computer monitor	Samsung	S22C350H
Excel (Microsoft Office Professional Plus)	Microsoft	v14.0.7106.5003
VLC Media Player	Video LAN	v2.1.2	Media player with playback speed modulation and video support
External Hard Drive	Western Digital	WDBAAU0020HBK-01	2 TB