Paw placement during walking is altered by analgesic doses of opioids and post-surgical injury in mice

Hind paw-directed assays are commonly used to study the analgesic effects of opioids in mice. However, opioid-induced hyper-locomotion can obscure results of such assays. We aimed to overcome this potential confound by using gait analysis to observe hind paw usage during walking in mice. We measured changes in paw print area following induction of post-surgical pain (using the paw incision model) and treatment with oxycodone. Paw incision surgery reduced the paw print area of the injured hind paw as the mice avoided placing the incised section of the paw on the floor. Surprisingly, oxycodone caused a tiptoe-like gait in mice, resulting in a reduced paw print area in both hind paws. Further investigation of this opioid-induced phenotype revealed that analgesic doses of oxycodone or morphine dose-dependently reduced hind paw print area in uninjured mice. The gait changes were not dependent on opioid-induced increases in locomotor activity; speed and paw print area had no correlation in opioid-treated mice, and other analgesic compounds that alter locomotor activity did not affect paw print area. Unfortunately, the opioid-induced 'tiptoe' gait phenotype prevented gait analysis from being a viable metric for demonstrating opioid analgesia in injured mice. However, this work reveals an important, previously uncharacterized effect of treatment with analgesic doses of opioids on paw placement. Our characterization of how opioids affect gait has important implications for the use of mice to study opioid pharmacology and suggests that scientists should use caution when using hind paw-directed nociceptive assays to test opioid analgesia in mice. ### Competing Interest Statement The authors have declared no competing interest.