Back Mechanical Sensitivity Assessment in the Rat for Mechanistic Investigation of Chronic Back Pain

Hatef Khosravi; Nasim Eskandari; Benjamin Provencher; Thierry Paquette; Hugues Leblond; Emad Khalilzadeh; Mathieu Pich&#233;

doi:10.3791/63667

JoVE Journal > Behavior

Sciences du comportement

Back Mechanical Sensitivity Assessment in the Rat for Mechanistic Investigation of Chronic Back Pain

Published: August 30, 2022

doi:

10.3791/63667

Hatef Khosravi^1,2, Nasim Eskandari^1,2, Benjamin Provencher^1,2, Thierry Paquette^1,2, Hugues Leblond^1,2, Emad Khalilzadeh^1,3, Mathieu Piché^1,2

¹Department of Anatomy,Université du Québec à Trois-Rivières, ²CogNAC Research Group,Université du Québec à Trois-Rivières, ³Division of Physiology, Department of Basic Sciences, Faculty of Veterinary Medicine,University of Tabriz

Summary

To develop novel therapeutic interventions for the prevention and management of back pain, animal models are required to examine the mechanisms and effectiveness of these therapies from a translational perspective. The present protocol describes the BMS test, a standardized method to assess back mechanical sensitivity in the rat.

Abstract

Low back pain is the leading cause of disability worldwide, with dramatic personal, economic, and social consequences. To develop novel therapeutics, animal models are needed to examine the mechanisms and effectiveness of novel therapies from a translational perspective. Several rodent models of back pain are used in current investigations. Surprisingly, however, no standardized behavioral test was validated to assess mechanical sensitivity in back pain models. This is critical to confirm that animals with presumed back pain present local hypersensitivity to nociceptive stimuli, and to monitor sensitivity during interventions designed to relieve back pain. The objective of this study is to lay down a simple and accessible test to assess mechanical sensitivity in the back of rats. A test cage was fabricated specifically for this method; length x width x height: 50 x 20 x 7 cm, having a stainless-steel mesh on the top. This test cage allows the application of mechanical stimuli to the back. To perform the test, the back of the animal is shaved in the region of interest, and the test area is marked to repeat the test on different days, as needed. The mechanical threshold is determined with Von Frey filaments applied to the paraspinal muscles, utilizing the up-down method described previously. The positive responses include (1) muscle twitching, (2) arching (back extension), (3) rotation of the neck (4) scratching or licking the back, and (5) escaping. This behavioral test (Back Mechanical Sensitivity (BMS) test) is useful for mechanistic research with rodent models of back pain for the development of therapeutic interventions for the prevention and management of back pain.

Introduction

Low back pain (LBP) is the leading cause of disability worldwide, which has dramatic personal, economic, and social consequences¹^,²^,³^,⁴. Every year, approximately 37% of the population is affected by LBP⁵. LBP usually resolves within a few weeks but recurs in 24%-33% of individuals, becoming chronic in 5%-10% of cases². To understand the mechanisms and impacts of LBP as well as the effects of different therapeutic interventions, several animal models of LBP have been used, mimicking clinical conditions or some components of LBP⁶. These mouse and rat models can be classified in one or more of the following categories: (1) discogenic LBP⁷^,⁸^,, (2) radicular LBP⁸^,⁹^,¹⁰^,¹¹, (3) facet joint osteoarthritis¹², and (4) muscle-induced LBP¹³^,¹⁴. Since the pain cannot be measured directly in non-human species, numerous tests have been developed to quantify pain-like behaviors in these models⁸. These tests assess behaviors evoked by a noxious stimulus (mechanical force¹⁵^,¹⁶^,¹⁷, thermal stimulation¹⁸^,¹⁹^,²⁰^,²¹^,²²^,²³^,²⁴^,²⁵) or produced spontaneously²⁶^,²⁷^,²⁸^,²⁹.

The methods using mechanical stimuli include the Von Frey test¹⁵^,¹⁶ and the Randall-Selitto Test¹⁷. Methods using heat stimuli include the tail flick test¹⁸, hot plate test¹⁹, Hargreaves test²⁰, and thermal probe test²¹. Methods using cold stimuli include the cold plate test²², acetone evaporation test²³, and cold plantar assay²⁴. Methods for spontaneous behaviors include the grimace scales²⁶, burrowing²⁷, weight-bearing and gait analysis²⁸, as well as an automated behavioral analysis²⁹. Despite these numerous available tests, none of them is designed specifically for back pain models.

The objective of this study is to lay down a simple and accessible test to assess mechanical sensitivity in the back of rats. The technique is largely based on the Von Frey test applied to the plantar surface of the hind paw¹⁵^,¹⁶. The basic principle of the Von Frey test is to use a series of monofilaments to the region of interest, delivering constant pre-determined forces. A response is considered positive if the rat shows a nocifensive behavior. The mechanical threshold can then be calculated based on the filaments that evoked responses. In the present study, a simple and accessible method adapted from the Von Frey test is provided to determine mechanical sensitivity in the back of rats.

Protocol

The experimental protocol was approved by the animal care committee of Université du Québec à Trois-Rivières and conformed to the Guidelines of the Canadian Council on Animal Care and the Guidelines of the Committee for Research and Ethical Issues of the International Association for the Study of Pain (IASP). The present study used six male Wistar rats (body weight: 320-450 g; age: 18-22 weeks). The animals were obtained from a commercial source (see Table of Materials). Data from these rats are from the larger sample of a previous study³⁰.

1. Experimental preparation

House the animals in a temperature-controlled room in standard animal facilities with access to food and water ad libitum and a light-dark cycle of 14 h-10 h. Ensure that all animals are in good health on the day of the experiments.
Generate the chronic back pain animal model following the steps below.
1. To induce chronic back pain, perform an intramuscular injection of Complete Freund Adjuvant (CFA) into the back muscles following the previous reports¹⁴^,³⁰^,³¹.
2. Anesthetize the animal using isoflurane (4% for induction and 2%-2.5% for maintenance).
3. Using a 27 G needle, inject 150 µL of a ready-to-use water-in-oil emulsion of CFA (see Table of Materials) into the paraspinal muscles unilaterally or bilaterally, depending on the protocol needs.
4. Keep the injection needle in place for at least 3 min after completing the injection. For animals in the control group, use the same procedures³⁰, but inject a solution of sterile physiological saline solution (150 µL, 0.9%) instead of CFA.
Fabricate the test cage.
1. Make a test cage for two animals that comprises one chamber for each animal.
  NOTE: For the present study, each chamber has the following dimensions: length x width x height: 50 x 20 x 7 cm (see Table of Materials).
2. Mount the two contiguous chambers on four 33 cm long Plexiglass legs. Use transparent Plexiglass for the walls of the chambers, but use black Plexiglass to separate chambers to prevent animals from seeing each other.
3. Use stainless-steel mesh made of 1 mm wire with an 8 mm inter-wire distance to make the floor and ceiling of the test cage (Figure 1).

2. Back Mechanical Sensitivity (BMS) test

Familiarize the animal with the test cage 30 min/day for 5-7 consecutive days prior to the first test. Repeat the test as needed.
Anesthetize the animals using 2% isoflurane³¹ (see Table of Materials).
In a prone position under isoflurane anesthesia, shave the back hair of the area of interest (from T6 to L6 vertebral levels) using an animal hair trimmer (see Table of Materials). For repeated measures, shave the back hair every 3 days on a day without behavioral assessment to ensure that stimuli are always applied directly to the skin. Draw a black mark on the skin with a permanent marker to ensure that filaments are always applied to the same area when repeating the test on different days.
On the testing day, put the animals in the test cage for 15-30 min prior to the test until the animal is calm.
During the test, apply Von Frey filaments (0.07, 0.16, 0.4, 0.6, 1, 2, 4, 6, 10, 15, and 26 g) perpendicularly to the back, always beginning with the 2 g filament and using the up-down method¹⁵ (see Table of Materials). Approach the back of the animal slowly with the filament from behind the animal.
1. Apply the filament only when the animal is awake, standing on its four paws, and not moving. Apply the filament for 2 s bilaterally to the area of interest, 10 mm from the spinous process (Figure 2), every 15-30 s.
  NOTE: A response is considered positive if the animal exhibits one or more of the following behaviors during or immediately after the filament is applied: (1) muscle twitching, (2) arching (back extension), (3) rotation of the neck to look at the back, (4) scratching or licking the back, and (5) escaping.
As described previously¹⁵, if no response is observed with the application of a filament, apply the next filament with a higher force in the series. If a response is observed, use the next filament with a lower force in the series. Continue this procedure until four readings are obtained after the first behavioral change (response after a series of "no response" or no response after a series of "response").
Once data collection is completed, calculate the value representing 50% of the mechanical threshold, as described by Chaplan et al.¹⁵, using this formula:
50% threshold (g) = 10^(Xf+kδ)/10 000
NOTE: In this formula, "Xf" is the Handle Mark of the last von Frey filament that was used. "k" is the tabular value based on the animal's response pattern¹⁵, and "δ" is the mean of Handle Mark's increments between Von Frey filaments. Depending on the experimental design and experimental needs, only one side of the spine may be assessed to report one threshold, or two sides may be evaluated, and thresholds are reported separately or as a mean. Refer to Supplementary Table 1 for the calculation template³².

3. Animal recovery

After the intramuscular injection is completed, discontinue anesthesia and place the animal alone in a standard housing cage for recovery.
During the recovery period, examine the animal's behavior and do not leave it unattended.
Confirm that the animal recovers from anesthesia and moves normally within 5 min. Then, return the animal to its usual housing cage with the other animals.
NOTE: At the end of the experiment, the animal is perfused through the heart with a 10% formalin solution, under deep isoflurane anesthesia (5%). The back muscles in the injected area are then extracted for histology and confirmation of inflammatory changes.

Representative Results

The method was used in a previous study, in which full data and statistics were presented to compare back mechanical sensitivity between CFA and control rats³⁰. Representative individual data (mean of left and right thresholds) from six rats included in the previous study are presented in Figure 3 and Table 1. At baseline, mechanical sensitivity was similar between groups. Intramuscular injection of CFA in the lumbar muscles caused a marked increase in mechanical sensitivity (decreased threshold) from 7 days to 28 days after CFA injection. In contrast, the control (CTL) rats did not show this change. As shown in Figure 3, variability was observed within and between animals, as expected with this type of behavioral assessment. However, hypersensitive CFA rats showed decreased variability. Based on the previous study³⁰, 16 animals (8 CFA and 8 CTL) is sufficient to detect a significant effect between groups over time (η²_p = 0.38) for 5 time points.

In this study, the presence of chronic inflammatory changes in the muscles injected with CFA was confirmed by histological examination (Figure 4)³⁰. Also, mechanical hypersensitivity was observed at the hind paw with a standard Von Frey test, in addition to the back (Figure 5)³⁰. In previous studies with the same back pain model, we showed increased spontaneous pain behavior and neuroinflammatory and neurophysiological changes¹⁴^,³¹. Indeed, licking behaviors were increased in CFA compared with control rats during the formalin test, and single-unit responses to noxious stimulation of the sciatic nerve were altered in the right amygdala³¹. In addition, NF-kB protein expression was increased in the spinal cord of CFA compared with control rats¹⁴. Together, the results of these studies validate this chronic back pain model, and the present study visually demonstrates how to confirm the presence of mechanical hypersensitivity in the back of this rat model.

Figure 1: Back Mechanical Sensitivity (BMS) test cage. (A) Schematic drawing of the test cage. (B) Custom-made test cage comprising two chambers, one for each animal. (C) Lateral view of the test cage with a rat in one of the chambers. Please click here to view a larger version of this figure.

Figure 2: Back Mechanical Sensitivity assessment. The experimenter approaches the animal from behind and applies the Von Frey filament to the area of interest, 10 mm laterally from the spinous process. Please click here to view a larger version of this figure.

Figure 3: Individual examples of back mechanical sensitivity. Back mechanical sensitivity in CFA and control (CTL) rats, at Baseline and 7, 14, 21, and 28 days after the intramuscular injection of CFA or saline, respectively. Individual data are shown by gray (CTL) and black (CFA) filled circles. Horizontal bars indicate the means. Error bars denote the standard error of the mean. Please click here to view a larger version of this figure.

Figure 4: Histological confirmation of chronic muscle inflammation. Individual examples of back muscles from CFA rats and controls³⁰. (A) Healthy back muscle from a control rat 14 days after intramuscular injection of saline. (B–C) Back muscles from two CFA-treated rats showed chronic inflammation 14 days after intramuscular CFA injection, with a clear leukocyte infiltration. Hematoxylin-eosin coloration was used for staining the muscle slices. Scale bar = 250 µm. Please click here to view a larger version of this figure.

Figure 5: Mechanical hypersensitivity in CFA rats³⁰. Time course of mechanical sensitivity over 4 weeks, following either CFA (n = 8) or saline (n = 8) injection into the back muscles (L5-L6 level). (A–B) Mechanical sensitivity under the left and right hind paws. Mechanical thresholds significantly reduced in CFA compared with control rats (P < 0.01) over time. This effect was not significantly different between left and right hind paws (P = 0.7). For both hind paws combined, the Tukey HSD test revealed lower mechanical thresholds in CFA compared with control rats, from 1 week to 4 weeks after injection (all P's < 0.03). Time courses for separate hind paws are shown for illustration purposes only (interaction not significant, see results for details). (C–D) Mechanical sensitivity in the back. Mechanical thresholds significantly reduced in CFA compared with control rats (P < 0.001) over time. This effect was not significantly different between left and right assessment sites (P = 0.3). For left and right assessment sites combined, the Tukey HSD test revealed a lower mechanical threshold in CFA compared with control rats, from 1 week to 4 weeks after injection (all P's < 0.05). Time courses for separate hind paws are shown for illustration purposes only (interaction not significant, see results for details). In panel (D), the individual data of one CFA rat is not shown at the baseline (9.6 g) for illustration purposes. Shaded areas represent baseline assessment. Please click here to view a larger version of this figure.

Rat	Group	Baseline	Day 7	Day 14	Day 21	Day 28
1		2.34	0.29	0.12	0.29	0.29
2	CFA	1	0.48	0.05	0.48	0.08
3		1.26	0.05	0.05	0.05	0.19
Mean ± SD		1.53 ± 0.58	0.27 ± 0.18	0.07 ± 0.03	0.27 ± 0.18	0.19 ± 0.09
4		1.59	2.61	0.64	3.26	2.45
5	CTL	1.15	0.63	3.41	2.3	1.29
6		0.43	1.26	0.77	0.32	2.09
Mean ± SD		1.06 ± 0.48	1.50 ± 0.83	1.61 ± 1.28	1.96 ± 1.22	1.94 ± 0.48

Table 1: Individual examples of back mechanical sensitivity in CFA and control rats.

Supplementary Table 1: Determination of mechanical threshold. This template table is used to calculate the mechanical threshold. The pattern of responses (X/O) is noted, and the values needed for the calculation are entered for Xf and k only, corresponding to the Handle Marking of the last filament that was used for the test and to the k-value associated with the response pattern, in this case, XX followed by OOXXO. Please click here to download this File.

Discussion

Critical steps
The BMS test is a simple method to assess mechanical sensitivity in the back of rats, either at one time point or repeatedly over days or weeks, when changes are expected to occur (pain models) or after pharmacological or non-pharmacological intervention. Critical issues of the method include the test cage, whose dimensions must ensure that the rat is comfortable but does not move too much. The animal's back must stay accessible through the mesh ceiling for reproducible mechanical stimulation. To limit variability in the threshold assessment, the back area under investigation must be shaved so that mechanical stimuli are applied directly to the skin. In addition, the skin needs to be marked for mechanical stimuli to be applied to the same area. Lastly, the experimenter must approach the filament to the skin from behind the animal to avoid being seen by the animal.

Compared with the Von Frey test used for assessing the mechanical sensitivity at the hind paw¹⁵^,¹⁶, the mechanical force needed to produce a positive response in the BMS test is lower. The filaments used for the test should be selected carefully. Using the following filaments should cover most experimental needs (0.07, 0.16, 0.4, 0.6, 1, 2, 4, 6, 10, 15, and 26 g) and prevent facing a ceiling or a floor effect. In this case, the 2 g filament is used for the first application. This can be adapted to experimental needs as long as the calculation is adjusted accordingly.

Modifications and troubleshooting
During a pilot experiment, the ideal area for the test was determined. Because of the shape of the rat's body, the thoracolumbar region is the most accessible area in the test cage. If there is no reason for the test to be performed in other regions of the spine, this is the area of choice for applying mechanical stimuli. The lumbar area is also easily accessible. When deciding which area to test, it must be kept in mind that the filament must be applied perpendicularly to the surface and bend properly to deliver the pre-determined calibrated force.

Limitations
The experimenter must be trained to observe the behaviors associated with the test. The five positive responses include muscle twitching, arching, neck rotation to look at the back, licking or scratching the back, and escaping³⁰. While most of these responses are easily observable, muscle twitching is sometimes subtle for stimuli of a lower force. Also, the rat may move spontaneously in the cage, so this must not be confounded with escaping, which occurs specifically when the filament is applied. To avoid confounding both behaviors, the experimenter must wait for the animal to be calm for at least a few seconds.

Significance and potential applications
Several rodent models of back pain are used in current investigations⁸. Surprisingly, however, no standardized behavioral test was validated to assess mechanical sensitivity in back pain models. This is critical to confirm that animals with presumed back pain present local hypersensitivity to nociceptive stimuli, and to monitor sensitivity during interventions designed to relieve back pain. The BMS test presented here provides a simple and accessible solution for these purposes. Although it was developed for rats³⁰, it may be adapted to other laboratory animals in the future.

Divulgations

The authors have nothing to disclose.

Acknowledgements

This work was supported by a grant from the Fondation Chiropratique du Québec and the Natural Sciences and Engineering Research Council of Canada (MP: grant #06659). The contribution of HK was supported by the Université du Québec à Trois-Rivières (PAIR program). The contribution of BP was supported by the Fonds de recherche du Québec en Santé (FRQS) and the Fondation Chiropratique du Québec. The contribution of TP was supported by the Natural Sciences and Engineering Research Council of Canada. The contribution of NE and EK was supported by the Fondation Chiropratique du Québec. The contribution of MP was supported by the FRQS.

Materials

Aerrane (isoflurane, USP) – Veterinary Use Only	Baxter	NDC 10019-773-60	Inhalation Anaesthetic ; DIN 02225875, for inducing anasthesia
Complete Freund Adjuvant (CFA)	Fisher Scientific	#77140	Water-in-oil emulsion of Complete Freund Adjuvant (CFA) with killed cells of Mycobacterium butyricum.
Male Wistar Rats	Charles River Laboratories		body weight: 320–450 g; age: 18-22 weeks.
Penlon Sigma Delta Vaporizer	Penlon	990-VI5K-SVEEK	Penlon Sigma Delta Vaporizer used for anasthesia
Sharpie Permanent Marker	Sharpie	BC23636	Permanent Marker, Fine Point, Black
Test cage	Custom-made		Width: 20 cm; Length: 50 cm; Height from the bottom to the top: 40 cm; Height from the bottom mesh to the top of the cage: 7 cm; Wall thickness: 5 mm; Mesh: 1 mm wire with an 8 mm inter-wire distance
Von Frey Filaments	Aesthesio, Precise Tactile Sensory Evaluator	514000-20C	Filaments from 0.07 g to 26 g
Wahl Professional Animal, ARCO Cordless Pet Clipper, Trimmer Grooming	Wahl	Kit #8786-1201	Animal hair trimmer, for shaving purposes, zero blade

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Khosravi, H., Eskandari, N., Provencher, B., Paquette, T., Leblond, H., Khalilzadeh, E., Piché, M. Back Mechanical Sensitivity Assessment in the Rat for Mechanistic Investigation of Chronic Back Pain. J. Vis. Exp. (186), e63667, doi:10.3791/63667 (2022).