Exercise-induced morphine insensitivity is accompanied with a decrease in specific G-protein subunits gene expression in rats

Abstract

Numerous studies have shown a decreased analgesic potency of morphine in exercise trained animals. Since G-protein subunits have a critical role in opioid effects at the cellular level and the exact mechanism(s) of exercise-induced morphine insensitivity has not been fully clarified, the present study was designed to determine the changes in the levels of Gαi and Gβ proteins gene expression involved in this phenomenon. All experiments were carried out on male Wistar rats. Nociceptive thermal threshold was determined by the tail-flick method. Physical training was performed using treadmill apparatus. Semiquantitative RT-PCR and Western blot techniques were used to estimate Gai and Gβ mRNA and protein levels, respectively and normalized to an internal standard (β-actin).The antinociceptive effect of intrathecal morphine (5, 10 and 15μg) was significantly suppressed in trained rats. Following physical training, a significant decrease in the mRNA and protein levels of Gαi (P<0.001 and P<0.05 respectively) and Gβ (P<0.01 and P<0.001 respectively) were observed in the dorsal portion of the lumbar spinal cord.In conclusion, our results reveal that the expression patterns of the opioid receptor signaling elements change in exercise training animals. This may be, at least partly, responsible for exercise-induced morphine insensitivity.