Dose–response of morphine on PWLs in hMOR-T, hMOR and saline or GFP rats. Different doses of morphine were given in 10 μl volume and the PWL responses were determined 30 min after each morphine application. The solid lines were the theoretical fit of the data using the Hill equation. A. The Hill coefficient (h) was 1.01 for GFP and 1.09 for saline rats. Morphine ED50 (μg/10 μl) was 3.23 ± 0.49 for GFP and 3.86 ± 0.53 for saline rats. (B) The h was 0.94 for hMOR-T, 1.02 for hMOR. The morphine ED50 was 1.08 ± 0.08 for hMOR-T, 1.38 ± 0.07 for hMOR. The apparent morphine affinities of hMOR-T and hMOR rats were significantly higher than that of saline or GFP rats.