(a, b) WT and MOR−/− mice were injected with saline (1 ml/kg) or morphine (20 mg/kg, 1 injection) and sacrificed after 30 min or 6 hr. Tissue homogenates (cortices and hippocampi) were run on a 10% SDS-PAGE followed by immunoblotting using an FHC-specific antibody. The bar graph shows FHC band density normalized to actin and represented with respect to the control (mean ± SEM from 3 separate immunoblots using a total of 3 animals/group); in WT animals FHC band density in the morphine-pretreated (6hr) mice is significantly higher than in the control (F2,15= 9.539, * P < 0.005). (c) Comparison between the basal levels of FHC in WT and MOR−/− mice: Cortical/hippocampal tissue homogenates prepared from MOR−/− mice showed no differences in FHC levels compared to WT (P > 0.05, n=6 animals/each group, i.e. WT and MOR−/−).