Name: functional and physiological methods of evaluating median nerve regeneration in the rat
Uploaded: 2020-04-18T14:00:00
Duration: 12 min 9 s
Description: Watch this Scientific Journal Video about Functional and Physiological Methods of Evaluating Median Nerve Regeneration in the Rat at JoVE.com

Diogo Casal received a grant from The Program for Advanced Medical Education, which is sponsored by Funda&#231;&#227;o Calouste Gulbenkian, Funda&#231;&#227;o Champalimaud, Minist&#233;rio da Sa&#250;de e Funda&#231;&#227;o para a Ci&#234;ncia e Tecnologia, Portugal. The authors are very grateful to Mr. Filipe Franco for the illustrative drawing in Figure 1. The authors would like to thank the technical help of Mr. Alberto Severino in filming and editing the video. Finally, the authors would like to thank Ms. Sara Marques for her help in all the logistical aspects pertaining to animal acquisition and maintenance.

The authors have nothing to disclose.

This paper presents a protocol to create different types of MN lesions and repair in the rat. Additionally, it illustrates how to evaluate the functional recovery of this nerve using several noninvasive behavioral tests and physiological measurements.
Notably, several of the functional tests described in this paper, namely the Ladder Running Test and the Rope Test, are significantly dependent on the rat&#8217;s willingness to perform the task with the expectation of obtaining the food reward51,52,53. It should be noted that certain rat strains are more amenable to training and performing reproducibly in this type of tests51,52,53. For example, Lewis rats perform poorly in these tests both in the training phase and subsequently51,52,53.
Rat housing should permit ample freedom of movement in agreement with their natural exploratory behavior, in addition to allowing experimental animals to get familiar with some of the elements present in the functional tests19. Therefore, different forms of housing allowing higher freedom of movement are shown. The big cages are personalized with enrichment elements that are later used in the functional tests (e.g., ropes and ladders).
Arguably, these enriching elements as well as the cages with incorporated running wheels and the individual training spheres provide a form of postoperative physiotherapy similar to that offered to human patients operated on the peripheral nervous system10.
Significantly, although some authors advocate dissecting the subcutaneous tissues and muscle fasciae bluntly or by clean cutting with a number 15 scalpel, the use of thermocautery when dissecting these structures is recommended to minimize the risk of postoperative hematoma.
It should be noted that numerous tests have been devised to test different aspects of peripheral nerve repair in the rat, namely axonal regeneration, target reinnervation, and functional recovery, some of which are beyond the scope of this study29,54,55,56. For example, kinematic analysis29,36,55 and histomorphometric assessment29,36,57 are widely employed by multiple authors. Additionally, several of these tests involve variations to maximize efficiency and/or reproducibility54. For example, mechanical algisemetry (i.e., evaluation of responses to mechanical painful stimuli) can be assessed qualitatively using a given von Frey filament, as described in the present paper, or semiquantitatively using successively stronger von Frey filaments, or even quantitatively using electronic devices that apply increasing pressures until a withdrawal response is observed30,54.
Similarly, although several authors use walking track analysis to evaluate forelimb nerve repair in the rat, other authors argue that single MN lesions frequently fail to produce reproducible changes in pawprints10,58,59. Furthermore, some have stated that these changes may not be proportional to muscle recovery10,60. Bearing this in mind, some researchers have advocated the use of walking track analysis in the forepaw mainly when assessing recovery after crushing neve lesions rather than after segmental nerve reconstruction10,50,61.
The Grasping Test is widely used to evaluate motor recovery of the muscles controlled by the MN16,27. To guarantee uniformity and reproducibility of the data obtained with this test, applying the Grasping Test using the well-established methodology proposed by Bertelli et al.16 is recommended. However, the current protocol differs in that it does not routinely immobilize the contralateral paw to prevent undue stress11,27. It should also be noted that other authors, after immobilizing the uninjured paw, quantitatively assess the Grasping Test using a dynamometer or a scale27,56. However, this quantitative evaluation may be affected by the strength the researcher applies to the rat&#8217;s tail26. Furthermore, it is difficult to distinguish between the strength generated by the digital flexor muscles (solely innervated by the MN in the rat and the object of the Grasping Test9) from the strength produced by the wrist flexors, which include the flexor carpi ulnaris that receives its innervation from the ulnar nerve9,10,27. In order to try to circumvent these potential biases, this protocol uses an ordinal scale similar to the Medical Research Council Scale commonly used to grade muscle strength in humans10,11,62. Alternatively, other authors have described detailed assessment of grasping using video analysis and a video-based scoring system11,63.
A potential disadvantage of using the MN compared to the sciatic nerve is that a greater amount of information is available regarding the latter nerve. This, in turn, can make comparison of data obtained with the MN with that of prior experimental works more difficult46,48,64. Additionally, the smaller size of the MN compared to the sciatic nerve makes surgical manipulation more challenging8,12,27,56,65.
Contrary to the methodology described in this paper, the electroneuromyography evaluation can be performed using transcutaneous monopolar electrodes placed in the arm and thenar regions51. Despite being less invasive, this method carries the risk of potential confusion due to the possibility of costimulation of the ulnar nerve in the arm region9,51.
Most authors concur that not all tests used in the rat provide concordant results, as peripheral nerve repair depends on a complex array of factors, comprising neuron survival, axonal elongation and pruning, synaptogenesis, successful recapture of the denervated sensory organs and motor units, and brain plasticity7,10,50,66,67.
Finally, it should be noted that a significant caveat of rodent models is that rat peripheral nerves are much closer to their end organs and have much smaller cross-sectional areas than the homologous human structures. However, this size difference guarantees faster experimental data in rodents, and better overall results in rats in comparison to humans are to be expected68. Indeed, several authors warn that care must be used when trying to extrapolate experimental data obtained in peripheral nerve repair using rodents to humans7,69. Primate models are considered more comparable70. Nevertheless, their use is associated with vexing ethical, logistical, and budget constraints71.
Even though the sciatic nerve is the most commonly used nerve in peripheral nerve research, the rat MN presents multiple advantages. For example, MN lesions are associated with a smaller incidence of joint contractures and automutilation of the affected paw11,12,16,56. Significantly, autotomy subsequent to sciatic nerve transection afflicts 11&#8211;70% of rats. This may make current evaluations like the sciatic index impossible14. This, in turn, makes the estimate of the number of animals required to obtain a given statistical power cumbersome15.
In addition, as the MN is shorter than the sciatic nerve, nerve recovery is observed sooner58,72,73,74,75,76. Furthermore, the MN is not covered by muscle masses, making its dissection technically easier than that of the sciatic nerve16. Additionally, the MN has a parallel path to the ulnar nerve in the arm. Hence, the ulnar nerve can easily be used as nerve graft for repairing MN injuries. Finally, in humans, most peripheral nerve lesions occur in the upper limb, which further supports the use of this nerve in the rat77,78.
Arguably, rodents are the experimental animals most commonly used in the realm of peripheral nerve repair48,79. As shown, the rat MN is a convenient model of peripheral nerve lesion and repair. In fact, there are multiple standardized strategies available to assess motor and sensory recovery, permitting an easier comparison of results36,46,60,80,81,82. Many of these methods are noninvasive, allowing for daily assessment.
Moreover, physiotherapy is part of the standard of care of patients recovering from peripheral nerve injuries. As demonstrated in this paper, there are multiple strategies to provide a postoperative physiotherapy-like environment to rats submitted to MN injuries4,5. Hence, this model is particularly suitable to replicate the clinical scenario, facilitating extrapolation of results to the human species12,27,48,56,58,83.
As shown in this paper, multiple standardized strategies are available to assess motor and sensory recovery in the MN model of the rat. The majority of these are noninvasive procedures, allowing frequent assessment. Moreover, as most peripheral nerve lesions in the human species occur in the upper limb, the mentioned experimental physiotherapy settings can more aptly mimic recovery in the clinical context. Arguably, this can facilitate extrapolation of results to the human species, further validating the use of this nerve in the rat.

Peripheral nerve lesions occur regularly as a result of trauma, infection, vasculitis, autoimmunity, malignancy, and/or radiotherapy1,2. Unfortunately, peripheral nerve repair continues to present clinically unpredictable and frequently disappointing results3,4. There is widespread consensus that considerable basic and translational research is still needed to improve the prospect of those affected4,5,6,7.
The rat MN shows great similarities to that of humans8,9 (Figure 1). Originating from the brachial plexus in the axillary region, this nerve descends into the medial aspect of the arm, reaching the elbow, and branching off to the majority of the muscles in the ventral compartment of the forearm. The MN reaches the hand, where it innervates the thenar muscles and the first two lumbrical muscles as well as to part of the rat&#8217;s hand skin9 (Figure 1).
Using the rat MN, it is possible to adequately replicate peripheral nerve lesions in humans10,11,12. This nerve has several potential research advantages relative to the customarily used sciatic nerve. Because the MN is located in the forelimb of rats (akin to the human upper limbs), it can be damaged experimentally with a much smaller impact on the rat&#8217;s wellbeing, compared to the sciatic nerve, which innervates a substantial portion of the pelvic limb13. Additionally, in humans most clinical lesions occur in the upper limb, which corresponds to the rat&#8217;s forelimb10,11,12,14,15,16.
This paper shows how to produce different types of MN lesions in the rat. Moreover, different ways to simulate postoperative physiotherapy are presented. Finally, tests to evaluate functional recovery of the MN are described. There are multiple standardized strategies available to assess motor and sensory recovery using an MN model of peripheral nerve lesion and repair, thus permitting an easy comparison of results. The MN model is particularly suitable to replicate the clinical scenario, facilitating extrapolation of results to the human species.

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</table>

functional and physiological methods of evaluating median nerve regeneration in the rat

The main goal of this investigation is to show how to create and repair different types of median nerve (MN) lesions in the rat. Moreover, different methods of simulating postoperative physiotherapy are presented. Multiple standardized strategies are used to assess motor and sensory recovery using an MN model of peripheral nerve lesion and repair, thus permitting easy comparison of the results. Several options are included for providing a postoperative physiotherapy-like environment to rats that have undergone MN injuries. Finally, the paper provides a method to evaluate the recovery of the MN using several noninvasive tests (i.e., grasping test, pin prick test, ladder rung walking test, rope climbing test, and walking track analysis), and physiological measurements (infrared thermography, electroneuromyography, flexion strength evaluation, and flexor carpi radialis muscle weight determination). Hence, this model seems particularly appropriate to replicate a clinical scenario, facilitating extrapolation of results to the human species.
Although the sciatic nerve is the most studied nerve in peripheral nerve research, analysis of the rat MN presents various advantages. For example, there is a reduced incidence of joint contractures and automutilation of the affected limb in MN lesion studies. Furthermore, the MN is not covered by muscle masses, making its dissection easier than that of the sciatic nerve. In addition, MN recovery is observed sooner, because the MN is shorter than the sciatic nerve. Also, the MN has a parallel path to the ulnar nerve in the arm. Hence, the ulnar nerve can be easily used as the nerve graft for repairing MN injuries. Finally, the MN in rats is located in the forelimb, akin to the human upper limb; in humans, the upper limb is the site of most peripheral nerve lesions.

A total of 34 rats were randomly divided into the following groups: Sham (n = 17), Excision (n = 17), and Nerve Graft (n = 10) for the operation. All rats survived surgery and the postoperative period uneventfully. One week after surgery and for the subsequent 100 days, all animals underwent the functional tests described above once a week. The representative results of each of these tests are described below.
Grasping Test
The percentage of rats with a positive response in the grasping test was highest for the Sham group. This value gradually increased over time in rats from the Crush and Nerve Graft groups (Figure 3).
Pin Prick Test
Rats from the Sham group had the best scores in the cumulative pin prick test relative to rats from the Nerve Graft group. Both had better scores than the rats in the Excision group (Figure 4).
Ladder Running Test
The rats&#8217; velocity in the ladder running test was highest in the Sham group than in the rats submitted to MN lesion. Among the latter, the time to run the ladder tended to decrease over time, paralleling MN recovery (Figure 5).
Rope Test
As in the ladder running test, the time the rats took to climb the rope was shorter in the Sham group compared to the groups in which the MN was injured. The rats&#39; speed in this test increased when the MN was allowed to recover (Figure 6).
Walking Track Analysis
Analysis of walking tracks tended to show changes in the morphology of paw prints (Figure 7). These changes were often more pronounced in crushing injuries than in segmental nerve lesions50.
Infrared Thermography
Thermography was useful when examining temperature differences between the forepaws in the first 30 days after surgery. Temperature differences were more noticeable in rats with a more severely injured MN, such as in those from the Excision group (Figure 8 and Figure 9).
Electroneuromyography
Table 1 summarizes the biological importance of the electroneuromyography measurements, providing representative results for the different experimental groups. Various patterns were observed with electroneuromyography. A normal CMAP was typical of a rat from the Sham group, while a polyphasic CMAP was associated with a variable degree of lesion of the MN, as in the Crush and in the Nerve Graft groups (Figure 10). In the Excision group, no CMAPs were observed.
Wrist Flexion Strength
Given that wrist flexion is mainly dependent on the MN, this test was used to evaluate motor recovery in this nerve&#8217;s territory. Wrist flexion strength was closest to normal when recovery was maximal (Figure 11).
Muscle Weight and Morphology
The weight and morphology of the flexor carpis radialis muscle were dependent on MN recovery, as this muscle is innervated exclusively by the MN9,10. Thus, normal weight and morphology were observed in the Sham group. A loss of weight and muscle trophism was observed in the Crush, Nerve Graft, and Excision groups (Figure 12).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59767/59767fig01.jpg" /> 
Figure 1: Schematic representation of the anatomy of the median nerve of the rat. 
(1) Origin and termination of the median nerve in the rat brain (green area = primary motor area; blue area = primary sensory area). (2) Transverse section of the spinal cord at C7 segment level; (3) Axillary nerve; (4) Musculocutaneous nerve; (5) Radial nerve; (6) Median nerve; (7) Ulnar nerve; (8) Medial cutaneous branch of the arm; (9) Medial cutaneous branch of the forearm; (10) Axillary artery; (11) Brachial artery; (12) Median artery; (13) Superficial radial artery; (14) Ulnar artery; (15) Motor branch of the median nerve to the pronator teres muscle; (16) Motor branch of the median nerve to the flexor carpis radialis muscle; (17) Motor branch of the median nerve to the flexor digitorum superficialis muscle; (18) Motor branch of the median nerve to the flexor digitorum profundus muscle; (19) Sensory branch of the median nerve to the thenar region; (20) Common palmar artery of the first interosseous space; (21) Radial palmar digital artery of the first digit; (22) Motor branch of the median nerve to the thenar muscles; (23) Palmar arterial arch; (24) Radial palmar digital nerve of the first digit; (25) Ulnar palmar digital nerve of the first digit; (26) Common palmar artery of the third interosseous space; (27) Motor branches of the terminal divisions of the median nerve to the first three lumbrical muscles; (28) Ulnar palmar digital nerves of the second, third, and fourth digits; (29) Ulnar palmar digital arteries to the fourth and fifth digits; (30) Radial palmar digital nerves of the second, third, and fourth digits; (31) Radial palmar digital artery of the fifth digit; (32) Skin territory of the median nerve in the forepaw (blue-shaded region). <a href="https://www.jove.com/files/ftp_upload/59767/59767fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59767/59767fig02.jpg" /> 
Figure 2: Photograph of the right forelimb of the rat showing the surgical anatomy of the median nerve in the arm and axillary regions. 
Cr, cranial; Me, medial <a href="https://www.jove.com/files/ftp_upload/59767/59767fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59767/59767fig03.jpg" /> 
Figure 3: Percentage of rats with a positive grasping test in the different experimental group over a period of 100 days after surgery. <a href="https://www.jove.com/files/ftp_upload/59767/59767fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/59767/59767fig04.jpg" /> 
Figure 4: Nociception evaluation using cumulative pin prick test results in the operated forepaw normalized to the contralateral paw in the different experimental groups. 
Vertical bars represent 95% confidence intervals. Horizontal lines in the upper part of the figure indicate statistically significant differences between experimental groups, ***p&#60;0.001. <a href="https://www.jove.com/files/ftp_upload/59767/59767fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/59767/59767fig05.jpg" /> 
Figure 5: Average speed in the ladder running test in the different experimental groups. 
Vertical bars represent 95% confidence intervals. Asterisks in the upper portion of the figure indicate statistically significant differences between groups, *p&#60;0.001. <a href="https://www.jove.com/files/ftp_upload/59767/59767fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/59767/59767fig06.jpg" /> 
Figure 6: Average climbing velocity in the rope test in the Sham and Excision groups. 
Vertical bars represent 95% confidence intervals. Asterisks in the upper portion of the figure show statistically significant differences between groups, *p&#60;0.05; **p&#60;0.01. <a href="https://www.jove.com/files/ftp_upload/59767/59767fig06large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/59767/59767fig07.jpg" /> 
Figure 7: Walking track parameters in the different experimental groups. 
Values on the operated limb are expressed as percentages of means normalized to the contralateral limb. (A) Stance factor; (B) Print length; (C) Finger spread factor; (D) Intermediate finger spread factor; (E) Stride length; (F) Base of support. Vertical bars represent 95% confidence intervals. Horizontal lines in the upper portion of the figure indicate statistically significant differences between experimental groups. D30, D60, D90 = 30, 60, and 90 days after surgery, *p&#60;0.05; **p&#60;0.01; ***p&#60;0.001. <a href="https://www.jove.com/files/ftp_upload/59767/59767fig07large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/59767/59767fig08.jpg" /> 
Figure 8: Mean temperature difference registered by infrared thermography. 
The box plots represent the temperature difference between the palmar region of the median nerve on the operated side (right side) and the contralateral side (left) in the Sham (n = 17) and Excision (n = 17) groups, *p&#60;0.05; **p&#60;0.01. <a href="https://www.jove.com/files/ftp_upload/59767/59767fig08large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 9" class="xfigimg" src="/files/ftp_upload/59767/59767fig09.jpg" /> 
Figure 9: Typical infrared thermography pattern of an animal from the excision group during the first 45 days after surgery. <a href="https://www.jove.com/files/ftp_upload/59767/59767fig09large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 10" class="xfigimg" src="/files/ftp_upload/59767/59767fig10.jpg" /> 
Figure 10: Typical patterns of Compound Muscle Action Potentials (CMAPs) from an animal from the Sham and Nerve Graft groups 90 days after surgery. <a href="https://www.jove.com/files/ftp_upload/59767/59767fig10large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 11" class="xfigimg" src="/files/ftp_upload/59767/59767fig11.jpg" /> 
Figure 11: Evaluation of wrist flexion strength on both forepaws 90 days postoperatively in different experimental groups. 
Wrist flexion strength was assessed using the area under the curve (AUC) over a time period of 30 s and using supratetanic stimulation. Vertical lines denote 95% confidence intervals. Horizontal lines in the upper portion of the figure highlight statistically significant differences between groups, **p&#60;0.01. <a href="https://www.jove.com/files/ftp_upload/59767/59767fig11large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 12" class="xfigimg" src="/files/ftp_upload/59767/59767fig12.jpg" /> 
Figure 12: Flexor carpi radialis muscle weight and macroscopic appearance 100 days after surgery. 
(A) Box plots depicting the normalized flexor carpi radialis muscle weight in different experimental groups, **p&#60;0.01; ***p&#60;0.001. (B) Photographs of the muscles on the right and left sides in the Sham and Excision experimental groups. <a href="https://www.jove.com/files/ftp_upload/59767/59767fig12large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Parameter</td>
			<td>Parameter significance</td>
			<td>Sham group</td>
			<td>Excision group</td>
			<td>NG group</td>
		</tr>
		<tr>
			<td>Neurological stimulation threshold (%)</td>
			<td>Evaluation of nerve regeneration, as there is a minimal number of nerve fibers required to produce either a CMAP or a visible muscle contraction12</td>
			<td>281.63 &#177; 271.65</td>
			<td>5359.98 &#177; 3466.52</td>
			<td>2108.12 &#177; 2115.13</td>
		</tr>
		<tr>
			<td>Motor stimulation threshold (%)</td>
			<td>Evaluation of nerve regeneration, as there is a minimal number of nerve fibers required to produce either a CMAP or a visible muscle contraction12</td>
			<td>462.52 &#177; 118.91</td>
			<td>1694.10 &#177; 503.24</td>
			<td>1249.50 &#177; 503.24</td>
		</tr>
		<tr>
			<td>Latency (%)</td>
			<td>Assessment of nerve conduction velocity in the fastest nerve fibers, that is to say the largest myelinated fibers44</td>
			<td>113.55 &#177; 25.04</td>
			<td>N/A</td>
			<td>132.80 &#177; 69.95</td>
		</tr>
		<tr>
			<td>Neuromuscular transduction velocity (%)</td>
			<td>Assessment of nerve conduction velocity in the fastest nerve fibers, that is to say the largest myelinated fibers44</td>
			<td>92.01 &#177; 20.88</td>
			<td>N/A</td>
			<td>91.30 &#177; 26.51</td>
		</tr>
		<tr>
			<td>CMAPs amplitude (%)</td>
			<td>Evaluation of the number of reinnervated motor units34</td>
			<td>110.63 &#177;45.66</td>
			<td>N/A</td>
			<td>41.60 &#177; 24.84</td>
		</tr>
		<tr>
			<td>CMAPs duration (%)</td>
			<td>Assessment of synchrony of muscle innervation, which is dependent on the degree of muscle reinnervation and myelination of the innervating motor fibers44,45</td>
			<td>101.12 &#177; 23.92</td>
			<td>N/A</td>
			<td>151.06 &#177; 54.52</td>
		</tr>
		<tr>
			<td colspan="5">NG, nerve graft 
			CMAPs, compound muscle action potential. 
			N/A, non-applicable 
			All parameters are expressed as percentages of the average contralateral values. 
			Numeric variables are expressed as average &#177; standard deviation.</td>
		</tr>
	</tbody>
</table>
Table 1: Electroneuromyographic assessment at the end of the experiment.

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