Effects of forced treadmill exercise during the induction of morphine dependence on the severity of physical and psychological dependence in morphine-dependent rats

Previous studies have indicated that the voluntary and swimming exercise decreases the severity of the physical and psychological dependence on morphine in morphine-dependent and withdrawn rats. The aim of the present study was to investigate the effects of the forced treadmill exercise during the induction of morphine dependence on naloxone-precipitated morphine withdrawal signs and anxiety like behavior.
Rats were injected with increasing doses of morphine (8, 16, 26, 36, 46, 56 and 64 mg/kg, daily, s.c.) over a period of 7 days in which they were also trained at mild intensity on a treadmill for 30 min of daily. The anxiety-like behaviors were tested 2h after receiving morphine injection (56 mg/kg) using the elevated plus-maze (EPM) on day 8. Then, the severity of morphine dependence was measured after an acute injection of naloxone (0.4mg/kg, IP), 2h after receiving morphine injection (56 mg/kg) according to a modified version of the Gellert–Holtzman scale on day 9.
The results showed that the withdrawal graded signs including abdominal contractions, weight loss, and overall Gellert–Holtzman score and among the checked signs; consisted of diarrhea, irritability and teach chattering were decreased in treadmill runner morphine-dependent rats than the sedentary rats. Also, the results showed that the treadmill runner morphine-dependent rats exhibited an increase in time spent in, and entries into, the EPM open arms than the sedentary morphine-dependent groups.
Our findings indicated that the forced treadmill exercise during the induction of morphine dependence diminished the severity of dependence on morphine and anxiety like behavior. Thus, forced treadmill exercise may decrease some of the behavioral consequences of physical and psychological dependence on morphine.