Meal Duration as a Measure of Orofacial Nociceptive Responses in Rodents

Summary

A lengthening in meal duration represents orofacial nociceptive behavior in rodents similar to the guarding behavior of humans with orofacial pain. Eating is a behavior that requires no training or animal manipulation, requires cortical participation, and is not competing with other experimentally induced behaviors, distinguishing this assay from alternative reflex or operant measurements.

Abstract

A lengthening in meal duration can be used to measure an increase in orofacial mechanical hyperalgesia having similarities to the guarding behavior of humans with orofacial pain. To measure meal duration unrestrained rats are continuously kept in sound attenuated, computerized feeding modules for days to weeks to record feeding behavior. These sound-attenuated chambers are equipped with chow pellet dispensers. The dispenser has a pellet trough with a photobeam placed at the bottom of the trough and when a rodent removes a pellet from the feeder trough this beam is no longer blocked, signaling the computer to drop another pellet. The computer records the date and time when the pellets were taken from the trough and from this data the experimenter can calculate the meal parameters. When calculating meal parameters a meal was defined based on previous work and was set at 10 min (in other words when the animal does not eat for 10 min that would be the end of the animal's meal) also the minimum meal size was set at 3 pellets. The meal duration, meal number, food intake, meal size and inter-meal interval can then be calculated by the software for any time period that the operator desires. Of the feeding parameters that can be calculated meal duration has been shown to be a continuous noninvasive biological marker of orofacial nociception in male rats and mice and female rats. Meal duration measurements are quantitative, require no training or animal manipulation, require cortical participation, and do not compete with other experimentally induced behaviors. These factors distinguish this assay from other operant or reflex methods for recording orofacial nociception.

Introduction

Animal models have been used to study pain and nociception associated with orofacial damage and or inflammation^1,2, but a lack of appropriate animal models results in an incomplete understanding of the mechanisms. Although current models help us to understand various mechanisms involved in acute and chronic orofacial pain, there are strengths and weaknesses to these animal models.

Many models measure behavioral nociceptive responses for short durations. Face grooming is a known behavioral response following constriction of facial nerves³. Others studies measured facial rubbing with the ipsilateral fore or hindpaw, as w....

Protocol

In this model the rats or mice were given food and water ad libitum. The Texas A&M University Baylor College of Dentistry Institutional Animal Care and Use Committee approved all the experimental protocols. Below specific settings are shown in italics and are utilized specifically for the rat TMJ arthritis model. Mice can also be utilized in this model and alternative tooth pain and myogenic orofacial pain animal models can be used as well^42,43.

1. Software Settings

1. Load the Animal Monitor software for the feeder units onto the computer.
2. The Animals Monitor software is now opened by clicking on ....

Results

Meal duration is a behavioral correlate of orofacial pain and meal duration measurements have been applied to animals with TMJ arthritis (Figure 2) and tooth decay (Figure 3). In one experiment, rats had TMJ arthritis after administering a high 250 mg dose of CFA and this treatment induced a significant increase in meal duration for 19 days (Figure 2). A lower dose of CFA (10 mg) injected into each TMJ joint produced a smaller increase in meal duration for only 2-3 days<.......

Discussion

TMJ patients with orofacial pain report increased pain with increased chewing time, such that, the chewing cycle lengthens the longer the individual has been chewing^45,53-56. Our behavioral assay allows for similar testing in rats and mice when measuring meal duration³⁹. A recent unpublished study suggested that von Frey filament testing had greater sensitivity than meal duration measurements, showing a significant change for a longer period but von Frey filament testing can have a reflex response c.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Animal Monitor software	Med Assoc. Inc	SOF-710	East Fairfield, VT
Dustless Precision Pellets, Rodent, Grain-Based	Bio-Serv	F0165	45 mg pellets, 50,000/box
Dustless Precision Pellets, Rodent, Grain-Based	Bio-Serv	FO163	20 mg pellets
Complete Freund's Adjuvant	Chondrex, Inc.	7001	No loger provides the 5 mg/ml concentration.  Can use CFA from other sources as long as the investigator consistently uses this source