Persistent exercise attenuates nicotine- but not clonidine-induced antinociception in female rats

Exercise decreases the antinociceptive effects of opiate drugs. It has been hypothesized that the exercise-induced attenuation of opiate drug action is the result of the development of cross-tolerance between endogenous opioids released during exercise and exogenous opiates. The present study was designed to evaluate the role of exercise on non-opiate antinociception. Female Long–Evans rats were allowed ad lib access to running wheels. After 3 weeks, antinociceptive responses of animals were measured using the tail flick test following the administration of clonidine or nicotine. Nicotine and clonidine both produced dose-dependent increases in antinociceptive responses. Active animals were significantly less sensitive to nicotine-induced antinociception than inactive animals. There was no difference between the two groups in clonidine-induced antinociception. The results of these experiments suggest that exercise does not attenuate non-opioid, clonidine-induced antinociception. However, exercise does attenuate nicotine-induced antinociception. Therefore, the effect of persistent exercise on analgesic drugs is not specific to opiates.