Focal Inflammation Causes Carbenoxolone-Sensitive Tactile Hypersensitivity in Mice

Regina Hanstein,David N Smith,Julie B Zhao,David C Spray,Menachem Hanani,Rajshekhar Basak,Maria Gulinello,Yonatan Y Zuckerman

doi:10.2174/1876386301003010123

Abstract

A focal and transitory inflammation induced by injection of complete Freund's adjuvant (CFA) in the submandibular skin of mice elicits pain behavior that persists for several weeks after the initial inflammation has resolved. Chronic pain, assessed as tactile hypersensitivity to stimulation with von Frey filaments, was evident from 1-7 weeks following CFA injection, although inflammation at the injection site was resolved by 3-4 weeks. In contrast, there were no changes in tactile sensitivity in the paw (un-injected site for comparison), no alterations in open field behavior and no differences in a functional observation battery evident in CFA-treated mice compared to controls (saline-injected) or to baseline (before CFA injection). Neither strain (Balb/c vs. C57BL/6) nor sex differences in baseline tactile threshold were significant in the submandibular skin. CFA-induced tactile hypersensitivity was also not a function of strain or sex. A single intraperitoneal injection of the gap junction blocker carbenoxolone (CBX) restored normal tactile thresholds in CFA-treated mice when administered at the peak of inflammation (1 week), after significant resolution of inflammation (3 weeks) or after total resolution of inflammation (4 and 5 weeks) without altering the tactile threshold of control subjects, tactile threshold in the paw or open field behavior. Thus, in this novel model of post-inflammatory pain, transitory inflammation induced persistent sex- and strain-independent behavioral hypersensitivity that was reversed by the gap junction blocker CBX, suggesting neuronal and/or glial plasticity as a major component of the chronic pain.

Highlights

Methods of inducing chronic pain in rodents generally fall into one of several categories: nerve damage, joint damage and/or inflammation and subcutaneous or intra-muscular injection of inflammatory or pain-causing agents
complete Freund’s adjuvant (CFA) and saline were injected into the midline of the submandibular skin, which is innervated by bilateral mandibular branches of the trigeminal ganglia
In our model of orofacial pain, inflammation was induced by the injection of complete Freund’s adjuvant (CFA) into the submandibular skin of Balb/c and C57BL/6 mice and total white blood cells were quantified over a 5-week time course (Fig. 1A-F). 1 week post injection all CFA-injected mice showed marked inflammatory cell infiltration in the submandibular skin, which was mainly comprised by polymorphic neutrophils and macrophages

Summary

Introduction

Methods of inducing chronic pain in rodents generally fall into one of several categories: nerve damage (axotomy, nerve crush, neuropathies), joint damage and/or inflammation (injection of adjuvants, formalin, etc.) and subcutaneous or intra-muscular injection of inflammatory or pain-causing agents (formalin, substance P, adjuvants, etc.). Each of these models has advantages, the injection of complete Freund’s adjuvant (CFA) into the submandibular skin is a new model with unique characteristics that provides several advantages. These characteristics may foster the dissection of the mechanisms and timing of events initiating and maintaining chronic pain without the complications of longterm inflammation and persistent stimulation that can confound the distinction between factors that engender and maintain chronic pain [6]

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Results

Conclusion