2.1. Animals and Complete Freund Adjuvant–induced inflammatory pain

All experiments were approved by the Institutional Care and Use Committees of Washington University in St. Louis and Washington State University, in accordance with the National Institutes of Health Guidelines for the Care and Use of Laboratory Animals. Male and female Long Evans rats (60–90 days) were pair housed with a 12/12 hours dark/light cycle and acclimated to the vivarium for at least 7 d before any manipulation. Rats received food ad libitum throughout the behavioral studies. We used the Complete Freund Adjuvant (CFA) model of inflammatory pain. Complete Freund Adjuvant (Calbiochem) was diluted in the same volume of sterile saline before its unilateral subcutaneous injection of 0.1 mL in the plantar surface of the right hind paw.^15,34 Behavioral experiments and tissue collection were conducted 48 hours after the injection of CFA. The 48-hour experimental time point is common across many studies of inflammatory pain, including our previous work.^1,16 Forty-eight hours is both sufficient for the development of sensory hypersensitivity and edema, as well as the earliest time point at which sensory hypersensitivity stabilizes and becomes persistent.