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

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 consequences^1,2,3,4. 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 co....

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

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 i.......

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 mechanica.......

Materials

Name	Company	Catalog Number	Comments
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