HSPA5 and Endoplasmic Reticulum Stress: On day 7, increased expressions of BiP mRNA (F2,11 = 20.01) and protein (F2,11 = 14.21) were detected in spinal cord of rats treated with morphine (Figures 1B–D, p < 0.01 compared to naïve and saline-treated rats), indicating that ER stress might be induced by chronic morphine treatment.