Effects of Forced Treadmill Exercise on Pain Threshold in Morphine-Addicted Rats
Alireza Sarkaki, Ph.D. Institute: Physiology Research Center â€“ Department of Physiology, Ahwaz Jondishpour University of Medical Sciences, Golestan Blv., Pin code 61375-15794, P.O. Box 45, Ahwaz, IR, Iran.Telefax: +98-611-3334009 and 3361544; e-mails: email@example.com and firstname.lastname@example.org
Background:Animal models comparing rat behaviours are often used in studies characterizing addiction and stress. Aim of this study was evaluation of five or ten days forced treadmill exercise effect on morphine addiction-induced hypoalgesia in young male rats.
Materials and Methods: In this study we used twenty four male Wistar rats weighing 200â€“300 g. Addicted and non-addicted rats have run as forced exercise on motorized treadmill one hour daily for ten days. Tail-flick latency was tested for each rat three times daily with 10 min intervals at a day before, 5 and 10 days after running on treadmill. A sham group consisted of animals placed on treadmill while its motor was off but electrical shock turned on. Mean of tail-flick latencies was analyzed statistically in sham, ran addicted and non-addicted rats.
Results: The tail-flick latencies were no significant alteration between all groups during 24 hours before forced running (1080 m distances daily). Animals ran 5400 m and 10800 m during 5 and 10 days on treadmill, respectively. Tail-flick latencies showed that pain reflex latency was increased significantly (p<0.001) in E.nA, nE.A, and exercised addicted group (E.A) groups in comparison to nE.nA rats after 5 and 10 days of addiction alone or with forced exercise, but it is significantly reduced in E.A vs. nE.A after 10 days of exercise (p<0.001).
Conclusion: Our data showed that treadmill forced exercise increased pain threshold in non-addicted rats, as well as morphine administration enhanced tail-flick latency in addicted groups after 5 and 10 days of exercise. This finding suggests that elevated stress hormones release followed by forced running and opioid receptor sensitivity associated with morphine administration could be the underlying reason why addicted runners have low pain threshold after 10 days of exercise.