Chronic morphine upregulates PLA2 activity. A, Dose-response relationship for AACOCF3 antagonism of DAMGO-mediated inhibition of IPSCs in neurons of the saline group (open circles) and of the morphine group (filled circles). N=6–12 cells for each data point. B, Similar experiments of AACOCF3 antagonism but in the presence of KT5720 for both the saline group (n=8–11) and the morphine group (n=5 for each AACOCF3 dose), showing AACOCF3 antagonism of PLA2-mediated DAMGO inhibition (normalized). C, D, Representative lanes and group data of Western blots for cPLA2 and phosphorylated cPLA2 as well as for β-actin in NRM tissues from placebo- (n=6 rats) and morphine pellet-implanted rats (n=9) (C), and from saline- (n=5 rats) and morphine-injected rats (n=5) (D). Data of % changes were normalized to the expression of β-actin. The molecular mass was 95 kD for cPLA2 and phosphorylated cPLA2. * p<0.05, ** p<0.01.