All procedures were approved by the Animal Care and Use Committee of Washington University and were in strict accordance with the U.S. National Institutes of Health (NIH) Guide for the Care and Use of Laboratory Animals. Adult male mice (8 to 14 weeks of age) used in experiments were housed in the Washington University School of Medicine animal facilities on a 12-hour light/dark cycle with access ad libitum to food and water.

For relevant optogenetic experiments, mice were generated with conditional expression of ChR2 in nociceptive sensory neurons by crossing heterozygous TRPV1-Cre mice [provided by M. Hoon (National Institute of Dental and Craniofacial Research)] (42) with homozygous Ai32 mice harboring ChR2 in the Rosa locus (stock no. 012569, the Jackson laboratory) (43). For the purposes of this study, we refer to these mice as “TRPV1-ChR2.” These mice are further characterized in the work of Park et al. (18). Heterozygous Ai32 mice (Cre-negative mice) were used as controls. All other experiments were performed using C57BL/6J mice bred in-house or obtained from the Jackson laboratory.