Gene expression at the mRNA level was assessed at the following seven time points: baseline, 3 hours of morphine exposure, 72 hours of morphine exposure, precipitated withdrawal (naloxone injection following 72 hours of morphine exposure), spontaneous withdrawal (18 hours following surgical removal of morphine pellets), abstinence (seven days following surgical removal of morphine pellets), and abstinence with forced swim (6 minutes of swim stress following seven days of abstinence). Placebo pellet controls were included for each experimental group.