Time course of morphine-induced analgesia. A dose of morphine (4 μg/10 μl) was given at time 0 (arrow) and PWLs expressed in (% MPE) in response to heat stimuli were measured at various intervals. The analgesia was observed within 15 min of morphine application and reached its maximum level at 30 min. The analgesia was gradually decreased and dissipated >100 min later. The MPEs obtained from hMOR-T and hMOR rats were significantly higher than that of saline rats (*p < 0.05, n = 5-7). The differences in MPEs between hMOR-T and hMOR rats were not significant.