Meal Duration as a Measure of Orofacial Nociceptive Responses in Rodents

The overall goal of this procedure is to determine that meal duration measurements are a quantitative correlate to jaw and tooth pain. This is accomplished by first placing the rats in feeder cages to get a baseline measurement of meal duration. Once baseline measurements are obtained, a pain inducing agent is injected into the temporal mandibular joint or knee joint as a control.

After the injection, the animal is returned to the feeder cage, and feeding behavior is observed until it returns to baseline levels. Ultimately, data are collected and analyzed to show that animals with jaw pain have longer meal durations. There are many advantages to using meal duration and meal patterns to analyze pain in rodent models, but we feel two of the main advantages of this model over other types of models is we don't have to restrain the animals, nor does the animal have to be trained.

Some years ago, we noted that there was not a good method to measure oral facial pain in an animal model. This led us to explore the possibility that meal pattern analysis could be used to measure oral facial pain. This led to development of the model that we've utilized today to measure oral facial pain.

Meal duration studies are performed in sound attenuated chambers equipped with computer activated pellet feeders add rodent chow to the food hopper consisting of 45 milligram pellets. These pellets are dispensed into the trough below that is equipped with a photo beam. There is a glass cover slip placed over the photo beam opening to prevent particles from blocking its path.

Once a rat removes the pellet from the trough, the photo beam is restored and the computer is signaled to drop another pellet. In this way, a pellet is always available without any additional effort from the animal. After filling the water bottle, place absorbent paper in the waste pan underneath.

To begin load the animal monitor software and select configuration from the pull down menu to set up the feeder units. Uncheck the pellet delivered input box to record when the pellet is removed from the trough instead of when it is dispensed. Next, check the automatic file naming box to automatically name the files.

After choosing the experiment option from the edit pull down menu, input the file name in the new window to ensure that the data are collected properly. Enter a value that exceeds the total time of the experiment in the experiment length days box. Next, set the number of hours in a day value to 24 for meal period and criteria.

Enter a value of 10 minutes. Enter 45 in the box entitled pellet size milligrams. In the phase section of the experiment window, enter the term day in the name field and in the number of hours field.

Type 24 as one example. Under the day phase header, enter the word light in the name field and enter 60 in the percent field. Repeat this procedure by entering dark in the name field with a percent value of 40.

Next, select the set all boxes like this button. Save this information and then hit okay. When the start boxes screen appears, select the feeders to activate and hit okay.

The animal monitor runtime windows will then appear with the meal pattern data. After observing baseline feeding behavior, remove the animal from the feeder for the induction of temporomandibular joint arthritis pain. Once anesthetized, locate the temporomandibular joint and inject complete fres adjuvant into the perticular space.

Repeat this procedure on the other side. Control rats are injected with 50 microliters of 0.9%saline. Another group of animals are injected with CFA in the knee joint.

After the injection, return the animals to the feeder unit as soon as they recover from anesthesia. During the meal duration assay monitor the feeding behavior of the arthritic rats until their meal duration returns to baseline levels. This can be from one to 19 days, depending on the dosage of CFA.

The software settings configured earlier will keep track of the number of pellets dispensed over a specific time period during the assay. Rinse and refresh the water bottles daily and add chow to the feeder hopper. When needed, replace the absorbent paper daily for the best results.

Blow the dust from the trough and moving parts of the feeder using high pressure air and wipe the cover slip clean daily. Clean the photo beam path if blockage is observed. At the completion of the study, remove and wash all of the cage components.

Files are generated daily and automatically open these files to retrieve meal pattern data such as food intake, meal number, or meal duration. Alternately, meal patterns can be calculated using proprietary software depending on the needs of the investigator in this experiment. Rats with CFA induced temporomandibular joint arthritis pain showed a significant increase in meal duration for 19 days when compared to saline injected animals or those receiving CFA injections in the knee.

This method can be applied to other forms of oral pain as shown here. Meal duration was also increased in rats experiencing pain following tooth pulp exposure. When using this procedure, there are two things that you need to troubleshoot probably daily.

When you use spray dolly rats of approximately 300 grams, they should eat between about 300 and 600 pellets a day. If they eat fewer pellets than say 300, there's probably an issue with the sensor at the top of the feeder unit that just needs to be cleaned. If they're eating more than 600 pellets, it's probably the sensor in the feeding trough and that would need to be cleaned.

So these are just two things you need to keep in mind when performing this assay. The meal pattern analysis model has several advantages over other models when studying oral facial pain. The model involves cortical pathways and thus is not measuring simply reflex mechanisms.

The model can be used 24 hours a day and does not invoke alternate behaviors. This model then becomes a very powerful tool in studying oral facial pain.