Background and Aim: Endogenous opioids and addictive opiate drugs change many body functions. . Previous studies have referred to the effects of morphine on smooth and pulmonary muscles ., but the  effects of opioids on skeletal muscles is not known well. Thus, the current study aimed at assessing the effect of a single dose of morphine on muscle fatigue in male rats.

Materials and Methods: In this experimental study, 40 male Wistar rats weighing 220-270 g were randomly divided into four equal groups: control (the mice were kept in their cages and received food and water), morphine receiving group, fatigue group (the mice in this group were kept running on  a treadmill . for120 minutes at a rate of 20 meters per minute), and morphine plus fatigue group. At the end of the experiments, blood samples were obtained from the corner of their eyes and were sent to the laboratory for measurement of muscle fatigue indexes including lactate dehydrogenase (LDH) and creatine phosphokinase (CPK).

Results: Administration of morphine to the fatigue group decreased running time compared with the control group (P=0.009). Furthermore, administration of morphine to the fatigue group significantly increased serum levels of LDH (P=0.009) and CPK (P=0.008).

Conclusion: The present study showed that administration of a single dose of morphine in rats increases muscle fatigue biomarkers (LDH, CPK).