A Modified Inflammatory Pain Model to Study the Analgesic Effect in Mice

Pain has a negative impact on quality of life. The method for evaluating pain plays a key role in developing anesthetic drugs. The present study tries to provide a reliable and superior method to evaluate analgesic effects.

The hot plate test is widely used to evaluate analgesic effects on inflammatory pain in mice. A commonly used method of inflammatory pain was induced with an intraplantar injection of carrageenan in one hind paw. We found that mice with a single hind paw carrageenan injection lifted their paws to avoid thermal nociception during the hot plate test.

Because of this response, the injection method cannot accurately reflect the thermal pain threshold. Thus, we modified the previous method by injecting carrageenan into both hind paws to establish the model of inflammatory pain. The results demonstrated that both-hind-paw injection with carrageenan was sensitive and a better method to induce inflammatory pain when used in the hot plate test than single-hind-paw injection.

In summary, we developed a superior method to induce a model of inflammatory pain to evaluate analgesic effects. The present experiment used only ICR mice. For test purposes in the future study, we will investigate whether this modified method accurately and consequently detects the pain threshold in the other mouse strains.

Begin by using ICR female mice weighing 20 to 25 grams for the experiments. Allow the animals to acclimate to the experimental room environment for three days before starting the experiments. To prepare carrageenan solution, weigh 0.1 grams of carrageenan.

Add 10 milliliters of sterilized normal saline to a tube. Add 0.1 grams of carrageenan to the tube, and vortex the tube for 30 seconds. Then collect the 1%sterilized carrageenan solution.

To begin, take a syringe to inject the prepared 1%carrageenan solution. Shake the carrageenan solution and draw it into the syringe while ensuring that there is no bubble in the syringe. Then disinfect the injection site with 75%alcohol.

Insert the needle diagonally upwards into the hind paw foot pad while maintaining the stability of the syringe to inject 30 microliters of 1%carrageenan solution. After injection, withdraw the syringe slowly and press on the injection site for two seconds to prevent leakage. clean and disinfect the surface of the hot plate.

Start the hot plate apparatus and click the temperature mode button. Then click constant to set the temperature to approximately 55 degrees Celsius. Click start experiment button and click reach.

Now take a mouse from the accommodation container. Gently grasp its trunk and place it in the center of the hot plate. Then click start.

Observe and record the time to lick the left hind paw or hind paw in response to the heat stimulation. Press the pedal of the hot plate to see the time on the hot plate screen. Then set a 60-second limit for the hot plate test.

Record a pain threshold of 60 seconds if the limit is reached. Take one capsule containing 200 milligrams of celecoxib. Get the content of the capsule and grind it into a powder.

To prepare the store suspension of celecoxib, add 0.5%sodium carboxymethylcellulose to make a final volume of 13.3 milliliters. Then take one milliliter of the store suspension of celecoxib and add 9.0 milliliters of 0.5%sodium carboxymethylcellulose to obtain celecoxib suspension. Administer 0.2 milliliters for 10 grams of the celecoxib suspension intragastrically to the mice.

Randomly divide ICR female mice into three groups, control, model, and celecoxib groups. Inject 30 microliters of 1%carrageenan solution subcutaneously into the planter surface of the hind paw. After the injection, intragastrically administer celecoxib at 30 milligrams per kilogram for the mice in the celacoxib group.

Conduct the hot plate test at three hours after carrageenan injection. The pain threshold of the single-hind-paw injection model did not show a significant reduction compared with the control group. The pain threshold of the both-hind-paw injection model decreased significantly compared with the control group.

In the single-hind-paw injection model, the pain threshold of the celacoxib group did not significantly increase compared to the model group. However, in the both-hind-paw injection model, the pain threshold of the celecoxib group showed a significant increase.